In chess, the "bishop pair" refers to one player having both of their bishops still on the board while the other player does not. The player with both bishops is said to "have the bishop pair" while the player without both bishops is said to have "lost," "relinquished," or "sacrificed" the bishop pair. Capturing one of your opponent's bishops while retaining both of your own is called "winning" the bishop pair.
The reason this concept arises is that although bishops are considered to be approximately equivalent in value to knights, bishops are known to be more valuable than knights in the endgame, and bishops are known to be especially more valuable than knights when a player has two of them working in concert, since once controls the light squares while the other controls the dark squares. When a player begins using both bishops in coordination to attack the opponent, this is often referred to as "unleashing" the bishop pair.