Well I'm no GM but for me there's a couple of things going on.

 1. In the given position white is threatening to take the pawn on a6
 2. If white can get a knight to d6 he will fork the rook on e8 and the bishop on c5, so winning the exchange after a move like Bd7

Thus black's move 23 protects the pawn on a6 by uncovering the line of the rook on a8. White then tries to get his knight to d6 via b7, and black stops the fork by moving his white squared bishop back to d7.