You just need to know the principle of bishop and rook pawn against king.
If your opponents pawn is a rook pawn (a or h file) and their bishop is the opposite colour of the promotion square, it's a draw. You just put your king in the corner.
So here, you're willing to sacrifice your bishop for one of the pawns (the h pawn since it queens on a light square).
Put your king on a1, and move your bishop backwards and forwards along one of the diagonals that controls a square the h pawn must pass through (the b8-h2 diagonal is probably best because it's longest so the king can't hassle you as much). As soon as the h pawn gets in sight of your bishop, take it and keep the king in the corner.
Your opponent can make no progress.
Hello,
I'm not able to finish the lesson.
Can someone explain this to me?
http://www.chess.com/computer-workout/server/opp-bs--2ps-no-problem
Thanks.