Weak Squares (Holes)
A weak square, or "hole", is a square that a side's pawns can never again defend, because every pawn that could have controlled it has already advanced past it. Since only pawns can permanently deny a square to enemy pieces (pieces can be driven away, but a pawn's control of a square is fixed by its position), once the pawns that would have covered a square move past it, that square is gone for good -- no future move can ever re-establish pawn control over it.
Holes matter most in the middle of the board (roughly ranks 3 through 6), where an enemy knight or bishop parked on a hole can become a long-term thorn that's extremely difficult to dislodge, since no pawn can ever attack it there. A knight planted on a deep hole, especially one supported by a friendly pawn, is the textbook definition of a powerful outpost.
This concept only really means something once there's still an actual pawn structure to speak of -- in a bare endgame with just a couple of pawns left, nearly every square trivially qualifies as a "hole" by the letter of the definition, which is technically true but not useful information. It's most meaningful earlier in the game, once a side has already committed several pawns forward.
Worked examples
Holes on d5 and e5
White's c- and f-pawns have already advanced past d5 and e5, onto c6 and f6 -- so those two squares can never again be covered by a white pawn.
Holes on d4 and e4 (Black's side)
Black's c- and f-pawns have already gone past d4 and e4, down to c3 and f3 -- the mirror image of the same idea, one rank set for the other color.
Test yourself
Click both holes in White's position.
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