The bishop pair refers to owning both bishops — the light-squared and the dark-squared — while the opponent has lost one or both of theirs.
This advantage is especially powerful in open positions, where long diagonals are unobstructed and allow the bishops to operate across the entire board. A single bishop controls only half the squares; both bishops together cover every square on the board, giving them greater mobility and versatility than most combinations of minor pieces (knight vs. bishop, or two knights). That said, this advantage is not absolute: in closed positions, where pawn chains block the diagonals, knights — which jump over obstacles — can outperform the bishops.
In practice, if you hold the bishop pair, aim to open the position by advancing central pawns or making exchanges that clear the diagonals. If your opponent holds it, keep the pawn structure closed and look for an opportunity to trade off one of their bishops to neutralize the advantage.