This move takes advantage of the fact that the d6-pawn is pinned by the white rook on d1. At the same time 1.Rxc5 clears the diagonal for the white bishop to play Bxc6 on the next move.
1.Rxc5 Qxc5 2. Bxc6 and white will capture one of the black rooks too.
1.Rxd6? would be a huge blunder because black will simply capture the white queen on a4, Nxa4.