Bishop pair
Two bishops cover both colours. In open positions the pair is worth roughly half a pawn over bishop and knight.
White to play
Bb2
Bb2 puts the second bishop on the second long diagonal. Between them they now cover both colours around the black king.
What to look for
Check whether you still have both bishops and they do not. If so, look for ways to open the position rather than to trade.
When it applies
In open positions with few pawns in the way. The pair's advantage is that between them they reach every square on the board, which only matters when there is room to travel.
How it goes wrong
In a blocked position with pawns fixed on both colours, a knight that can jump over the mess is worth more than either bishop. The pair is an advantage you have to open lines to collect.
Where it came up
Not cited in your current game.
Practise it
No positions yet. When you miss this idea in a game, the position it cost you shows up here — and in the practise set — with the answer hidden.