i've read people talking about weak squares. i don't understand it.
Yeah, most authors don't explain the basics very well.
The progression of a game is something like development -> open lines -> infiltration.
Why is infiltration useful? Well of course, if your pieces are on the opponent's side of the board, it's easier / more likely that they're in contact with targets in the opponent's camp, including the king.
A weak square is useful because once occupied, it attacks the surrounding squares.
And in the same way that even an isolated pawn is not called weak if nothing can attack it, a square wont be called weak if occupying it (or using it to infiltrate) is not useful.
Well, he leaned in close like a crazy person
so yeah, it was a single square.