Weak squares are created when pawns advance and leave behind holes in the pawn structure. For example, if White pushes the f-pawn to f4, the e3 and g3 squares may become weak if there are no pawns on adjacent files to cover them. An opponent can exploit these squares by placing a knight or other piece on them.
The most damaging weak squares are those in front of the king or in the center of the board. A weak square on f3 near a castled king can become an entry point for enemy pieces, while a weak square on d5 or e5 can serve as a dominant outpost for a knight.
Preventing weak squares requires careful pawn play: avoid unnecessary pawn advances, especially near the king, and think ahead about what squares will be left unprotected. When you identify a weak square in your opponent's position, plan to occupy it with a well-supported piece — preferably a knight, which cannot be chased away by an enemy bishop.