“The day is coming when you will see the sacrilegious object that causes desecration standing where he[13:14b Or it.] should not be.” (Reader, pay attention!) “Then those in Judea must flee to the hills. (Mark 13:14 NLT)
14 But when ye shall see the abomination of desolation, spoken of by Daniel the prophet, standing where it ought not, (let him that readeth understand,) then let them that be in Judaea flee to the mountains: (Mark 13:14 KJV)
In that verse Jesus is speaking to his disciples about "the time of the end" [Dan 12:4], of the future, I believe of the desecration of the Temple by the antichrist, before/during(?) the Tribulation. What, exactly, the "sacrilegious" object will be, I am unsure, but is likely some form of pagan idol or symbol. The NLT words that passage differently than the KJV but my Bible app offers some additional references to what Jesus says, citing these verses from Daniel about the "abomination of desolation":
Daniel 9:27 NLT -
The ruler will make a treaty with the people for a period of one set of seven, [9:27a Hebrew for one seven] but after half this time, he will put an end to the sacrifices and offerings. And as a climax to all his terrible deeds,[9:27b Hebrew And on the wing of abominations; the meaning of the Hebrew is uncertain] he will set up a sacrilegious object that causes desecration,[9:27c Hebrew an abomination of desolation] until the fate decreed for this defiler is finally poured out on him."
Daniel 11:31 NLT -
"His army will take over the Temple fortress, pollute the sanctuary, put a stop to the daily sacrifices, and set up the sacrilegious object that causes desecration. [11:31 Hebrew the abomination of desolation.]"
Daniel 12:11 NLT -
"From the time the daily sacrifice is stopped and the sacrilegious object that causes desecration[12:11 Hebrew the abomination of desolation.] is set up to be worshiped, there will be 1,290 days.
The answer to "standing where it should not be", would be the rebuilt (or partially rebuilt) Temple in Jerusalem.