I didn't understand either but I just looked it up and I think it works like this: if your opponent moves a pawn two squares up on the first move, and that skips the square that would make it vulnerable to be taken by your pawn, you can move your pawn to the square that their pawn would be at if they only moved it one square and take their pawn. According to Wikipedia "the rule ensures that a pawn cannot use its two-square move to safely skip past an enemy pawn"