Definition:
A hole is a square that cannot be guarded by a Pawn. Holes are most troublesome when they arise on a player's own 3rd and 4th ranks.
For example, if White plays 1.e4 and 1.c4, the square d4 is a hole. It might eventually be guarded by bxc3 or gxe3, but Black can play to avoid this if required.
A hole is a serious weakness in the Pawn structure because the opponent can play a piece to that square knowing that the piece cannot be driven away by a Pawn. A player tries to create a hole in the opponent's Pawn structure and then move a piece to the hole. At the same time the player tries to prevent holes from being created in his own Pawn structure.
