From the Catholic Editions of the Bible, the multitude means the Church, the spiritual Israel, the assembly belonging to Christ and to the Father, marked as and are the children of God, the people of God on earth praising God.
The Footnote to Revelation 7:4 Revised Standard Version Catholic Edition (RSVCE) has:
hundred and forty-four thousand: A symbolic number, i.e., twelve (the sacred number) squared and multiplied by 1,000 to denote a multitude. It is the church, the spiritual Israel, that is meant.
And the Navarre Bible New Testament Compact Edition note on Rev 14:1-5 has in part:
[...] The assembly includes all those who belong to Christ and to the Father and who therefore bear the mark which shows them to be children of God. They are so numerous that it is impossible to count them, but their number is defined and God has it in mind - the people of God represented by a number which is the result of multiplying 12 (tribes) by 12 (apostles) by 1000 (standing for a number on a huge scale); cf. 7:3-8. They are not yet in heaven; they are on earth, where they praise God, uniting their voices to the praise of the heavenly liturgy.
The footnote to Rev 7:4 in The Jerusalem Bible Popular Edition has:
Twelve (the sacred number) squared and multiplied by a thousand, representing the totality of the faithful.
Who are the 144,000 according to the Roman Catholic Church?
7 After these things, I saw four angels standing on the four corners of the earth, holding the four winds of the earth, that they should not blow upon the earth, nor upon the sea, nor on any tree.
2 And I saw another angel ascending from the rising of the sun, having the sign of the living God; and he cried with a loud voice to the four angels, to whom it was given to hurt the earth and the sea,
3 Saying: Hurt not the earth, nor the sea, nor the trees, till we sign the servants of our God in their foreheads.
4 And I heard the number of them that were signed, an hundred forty-four thousand were signed, of every tribe of the children of Israel.
5 Of the tribe of Juda, were twelve thousand signed: Of the tribe of Ruben, twelve thousand signed: Of the tribe of Gad, twelve thousand signed:
6 Of the tribe of Aser, twelve thousand signed: Of the tribe of Nephthali, twelve thousand signed: Of the tribe of Manasses, twelve thousand signed:
7 Of the tribe of Simeon, twelve thousand signed: Of the tribe of Levi, twelve thousand signed: Of the tribe of Issachar, twelve thousand signed:
8 Of the tribe of Zabulon, twelve thousand signed: Of the tribe of Joseph, twelve thousand signed: Of the tribe of Benjamin, twelve thousand signed. - Revelation 7:1-8
14 And I beheld, and lo a lamb stood upon mount Sion, and with him an hundred forty-four thousand, having his name, and the name of his Father, written on their foreheads.
2 And I heard a voice from heaven, as the noise of many waters, and as the voice of great thunder; and the voice which I heard, was as the voice of harpers, harping on their harps.
3 And they sung as it were a new canticle, before the throne, and before the four living creatures, and the ancients; and no man could say the canticle, but those hundred forty-four thousand, who were purchased from the earth.
4 These are they who were not defiled with women: for they are virgins. These follow the Lamb whithersoever he goeth. These were purchased from among men, the firstfruits to God and to the Lamb:
5 And in their mouth there was found no lie; for they are without spot before the throne of God. - Revelation 14:1-5
The number 144,000 simply is a symbolic number for the number of elect in Heaven. As a general rule, there is no official interpretation of this prophecy within the Church as it is not interpreted on an official level as prophecy may not be truly understood until it has actually run it’s course.
Occasionally the Bible will use the number one thousand to indicate not literally one thousand but an extremely large number. For example, when Revelation 20:6 says Christ will reign for one thousand years, it does not necessarily mean exactly one thousand years, but a ridiculously long period of time. Today, we express something similar when we say, “That was like a million years ago.” We are not expressing an exact measurement of time, but a long period of time. Similarly, in Revelation 7, John sees a vision of 144,000 servants of God, 12,000 from each tribe of Israel. This does not mean only 144,000 will be saved. It is a symbolic representation of the fullness of Israel, which will be saved plus the innumerable multitude from ever nation, race, people, and tongue (Rev 7:1-9). - The Meaning of Symbolic Numbers
Even the Catholic Encyclopedia passes over it quite quickly. This is not a hot topic.
The seven seals and the numbering of the saints (chapters 6 and 7)
At the opening of four seals, four horses appear. Their colour is white, black, red, and sallow, or green (chloros, piebald). They signify conquest, slaughter, dearth and death. The vision is taken from Zechariah 6:1-8. At the opening of the fifth seal the Seer beholds the martyrs that were slain and hears their prayers for the final triumph. At the opening of the sixth seal the predestined to glory are numbered and marked. The Seer beholds them divided into two classes. First, 144,000 Jews, 12,000 of every tribe. Then a numberless multitude chosen from all nations and tongues. - Apocalypse (Catholic Encyclopaedia)
Here follows the most common interpretations within the Catholic Church. In all the Church takes the number 144,000 as a symbolic number.
The 144,000 are the Church in its entirety, but especially in the age of doctrinal development, since to be protected physically is absurd. Jesus never promised that we would get out of suffering, but the contrary. We must take up our cross and follow Him. Miracles are exceptions. Hence, the protection of that Seal of God is spiritual. And what? To protect the forehead, the mind, from error, for that protection is always willed by God, and it is guaranteed if we but remain in the Church and heed her proclamations of truth.
As for 144,000, it is 12 x 12 x 1000
12 tribes of Israel, x 12 Apostles = fullness of Old and New x 1000 = great multitudes.
Now that's meaningful.
The Old Kingdom flowered into the New, and found its fulfillment. Hence, the fullness of all the kingdom of God, the New. Now that we have 12 Apostles indicates Apostolicity, which is authority, which in turn is realized in Christian division.
Moreover, that Scripture associates the 12 apostles with the twelve tribes seems to imply a mystical connection. - The 144,000 Apocalypse Thing
The B. C. Catholic explains it similarly, a little differently.:
Strictly speaking, Catholics do believe in a form of “rapture:” a bodily assumption into heaven of all the faithful, both living and dead, at Jesus' second and final coming and judgement. This will be preceded by the appearance of the Antichrist and an associated period of intense persecution of Christians known as the tribulation.
Thousand year reign?
But what about Jesus’ thousand year reign spoken of in Rv 20:4? Rapturists argue that if Jesus really did establish his kingdom on earth when he first came some 2,000 years ago, it should be over by now. They insist this period of time must be a literal thousand years.
The Church would answer that Revelation is a highly symbolic book and uses numbers and periods of time symbolically throughout. The number of the beast, 666, in Rev 13:8 is a perfect example of this, as is the 144,000 of Rv 14:3-5 (think 12 tribes representing the Old Testament, multiplied by 12 Apostles representing the New Testament, multiplied by 1,000). One thousand is a Biblical number that typically represents large, unspecified amounts or periods of time.
Christians usually understand Jesus’ thousand year kingdom reign in Rv 20 as a symbolically long period of time instituted by Jesus at his first coming and happening right now. - Do Catholics believe in the rapture?
The following articles may be of interest and may be perused at one’s leisure: