A weak square in chess is a square that can no longer be defended by a pawn of the side that owns it, allowing the opponent to occupy it with a piece that cannot be driven away.
A square becomes weak when the pawns that could have controlled it have advanced, been exchanged, or were never there. For instance, if White’s pawns are fixed on e4 and g4, the squares f3 and f5 can no longer be protected by any White pawn — Black can place a knight or bishop on f5 and it will remain firmly anchored. The more active and centralised the piece planted on that square, the greater the positional advantage it provides.
In practice, before pushing a pawn, ask yourself which squares it leaves permanently unguarded. Pawns cannot move backwards: every advance is a commitment, and it can create lasting weaknesses on the squares left behind.