hi tall, it's centred around these verses:
2Th 2:3 Let not anyone deceive you by any means. For that Day shall not come unless there first comes a falling away, and the man of sin shall be revealed, the son of perdition,
2Th 2:4 who opposes and exalts himself above all that is called God, or that is worshiped, so that he sits as God in the temple of God, setting himself forth, that he is God.
And there's many others but I let others post them.
A general belief is that there will be one man, called the anti-Christ, who will come to power as a sort of world dictator during the end-times tribulation. How and when that happens is the source of disagreement amongst christians.
But technically, anyone who is "against Christ", is an anti-Christ. In 1 John, the term was used to refer to the gnostics, who were an ancient sect that didn't believe Jesus actually came in the flesh but was only a ghost or spirit.