What does anti-positional mean?

Well, the way I understand it an anti-positional move is a move that creates a long term weakness in your position. It is typically related to your pawn structure. Your opponent may not be able to take advantage of it immediately but it can cause you trouble down the road.
An anti-positional move may or may not be a mistake. If you have compensation for the weakness you've created, e.g. through increased piece activity or initiative, then it can be a good move.

I was watching an agadmator video this morning, and he used the phrase anti-positional, and I had the same questions as the OP. So -- thanks for asking, and thank you to those who answered! Now I'm going to go back to where he said it and see if it makes more sense.
FYI, it at the 2:15 mark in this video: https://youtu.be/gPJibSgeFew
BTW, he added, "but nowadays with strong engines, players are making anit-positional moves all the time."