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.