A bad bishop is one whose own pawns are fixed on the same colour squares it travels on, blocking its diagonals and leaving it passive.
A bishop only moves on squares of one colour. If your own pawns get stuck on that colour, they wall the bishop in — it has few squares to go to and can’t influence the squares its pawns sit on, so it watches the game rather than playing in it.
The classic example is a light-squared bishop trapped behind a fixed pawn chain on light squares. The bishop ends up defending pawns instead of attacking anything, while the opponent’s pieces roam freely.
Two cures: trade the bad bishop off for a more active enemy piece, or change the pawn structure so the bishop’s diagonals open up. Until then, treat it as a long-term weakness and play around it.
Its own pawns are fixed on the same colour squares it moves on, blocking its diagonals so it has little scope and ends up defending pawns rather than attacking.
Either trade it off for an active enemy piece, or break up the pawns blocking it so its diagonals open. A bad bishop that gets traded stops being a problem.
BetterChess is a practice tool — we make no guarantee you'll reach 1800 or any rating. Definitions are standard chess terminology; every diagram position is checked legal with the same engine the board runs.