Opposite-colored bishops describes a situation where each side has one bishop travelling on squares of a different colour: one moves only on the light squares, the other only on the dark squares. The two bishops can never confront each other or defend the same squares.
In a bishop endgame, this configuration shows a strong drawing tendency: the defending side places its king and pawns on the squares the enemy bishop cannot control, sets up a blockade and often a genuine fortress. An extra pawn — sometimes two — is then no longer enough to win: the theoretical draw is never far away.
In the middlegame the effect reverses: opposite-colored bishops favour the attacker, because the defender cannot use his own bishop to cover the squares under attack. Combined with major pieces (queens, rooks), they fuel the initiative rather than the draw. Knowing which side of this switch you are on — a pure endgame or an attack with major pieces — is an essential evaluation landmark.