A good bishop is a bishop whose diagonals are not obstructed by its own pawns, allowing it to move freely and exert maximum influence across the board.
A bishop becomes "good" when most of the friendly pawns are placed on squares of the opposite color to the bishop. This means the pawns do not block its diagonals, and the bishop can roam actively to attack, defend, or control key squares. By contrast, a bishop hemmed in by its own pawns on the same color is called a "bad bishop" — it is locked behind its own pawn structure and contributes little to the position.
In practice, to keep your bishop active, place your pawns on squares of the opposite color from your bishop as early as the opening and middlegame. If you have a good bishop against your opponent’s bad bishop, look to open diagonals and create passed pawns that your bishop can support, while avoiding pawn advances that would block your own piece.