Good bishops and bad bishops

A bishop is permanently stuck on squares of one color -- light or dark. Whether that's a strength or a weakness in a given position depends heavily on where your own pawns sit. If most of your own pawns are fixed on the same color square as your bishop, they physically block its diagonals and get in its own way; that's called a 'bad' bishop. If most of your pawns sit on the opposite color, the bishop's diagonals stay open and it can range freely; that's a 'good' bishop.

The classic example is the French Defense, where black's central pawn chain often ends up on light squares while black's remaining bishop is also light-squared -- boxed in by its own structure, and a long-running theme of the whole opening.

This judgment is only made once a side still has a real pawn structure left -- three or more pawns -- since in a near-empty endgame almost any bishop can get around, and the label stops meaning much.

A bad bishop isn't necessarily a lost cause: pawns can advance and change which squares they occupy, pieces can be traded to open lines, and sometimes a 'bad' bishop is actually doing a great defensive job precisely because it sits on the same color as the pawns it's protecting. But all else equal, the side with the freer bishop has an easier time coordinating their pieces.