Builds a drawing fortress with opposite-coloured bishops.
When one bishop runs on light squares and the other on dark squares, they are opposite-coloured bishops. In these endgames even two extra pawns are often not enough, because the defender stops the pawns on the colour of its own bishop.
Black is two pawns down. White wants to push a pawn to f6. Take control of f6 with your bishop and build the fortress.
Practical lesson: when you are behind, escaping into an opposite-coloured bishop ending often saves half a point. When you are ahead, try to avoid it.