A weak square (or ‘hole’) is a square that can no longer be defended by any of your pawns, so an enemy piece can settle there safely.
Every pawn move is permanent, and pushing a pawn gives up control of the squares it used to guard. When neither neighbouring pawn can ever cover a square again, that square becomes a hole.
A hole matters most when it’s near your king or in the centre, because the opponent can plant a piece there — often a knight — that you can’t challenge with a pawn. The square then becomes an outpost for them.
Weak squares usually come in colour complexes: trade off a fianchettoed bishop, for example, and all the squares of that colour around your king can go soft at once. Think before every pawn push about what it permanently concedes.
A weak square is an empty square pawns can’t defend; a weak pawn is a pawn that’s hard to defend, like an isolated or backward pawn. They often appear together.
Yes — ‘hole’ is just the common informal name for a square that can no longer be guarded by a pawn.
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