Basically something is weak when a player can potentially bring more attackers than the opponent can bring defenders. Or when the defense is so inconvenient it's not really worth it (if you had to bring all your pieces to the back of the board to save a pawn it's not worth the pawn for example).
Sometimes it's geometric... they can bring 3 attackers in 4 moves and you can bring 3 defenders in 6 moves.
Sometimes it's an inherent limitation of the pieces. For example your light square bishop can't ever support a dark square, and pawns can't move backwards to support squares they've left behind. Or more loosely, if you have a piece physically blocked form moving to other other side (perhaps pawns are in the way).
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A caveat though, if the weakness (as defined above) isn't important then the annotator wont mention it. For example the square a2 may often be weak under the above definition... but if infiltrating on a2 achieves nothing then both players will ignore it.
Central squares and squares near either king that lack support are often important though.
Hi guys, I'm a beginner. Can someone tell me how to identify weak squares and weak pieces. Thanks.