There are a couple of good ways for White to play this position. Start here: what does White currently threaten? His bishop attacks Black’s knight, and his rook attacks Black’s bishop—behind which is the knight. This last point—the two Black pieces lined up with White’s rook aimed through them—suggests a pin. Well, but it might seem that the bishop isn't yet pinned, since if it moves the knight on b8 is protected against capture. Then again, the knight already is attacked by White’s bishop, too; so if the Black bishop on b5 moves, the knight on b8 finds itself protected once but attacked twice. So the b5 bishop is pinned.
But taking advantage of the pin is another matter. Imagine an attack against the bishop with the pawn push c3-c4. Black has a great reply: e7-e5, blocking the White bishop’s path to b8 and threatening to take it next move if White plays c4xB. So the pin doesn't quite work, because it depends on multiple lines of attack against b8—and one of those lines can be disrupted.
Yet White can fix the problem easily by reversing his order of operations: first he exhausts the threat against Black’s knight with BxN. Black recaptures RxB. Now see how things are different: Black's bishop on b5 blocks an attack against a loose piece; in other words, White has a good old fashioned relative pin in place. The pawn push that didn't work a minute ago—c3-c4—now works fine. Black can’t help losing a piece. (It would be different if the pinned piece were a dark squared bishop, because then it could leave the b-file and perhaps still protect the rook; but it isn’t.)
There is, as mentioned before, another direction for White to take here, and it works even better. Begin by noticing how close White is to delivering mate. Look at his checks and you find Ra1. You see that b4 won't work as a flight square for Black's king; it would have to move to b3. So White can begin by playing Nd2, which doesn't look very menacing at first but actually is quite scary for Black; the knight now seals off b3, so Black faces mate on the next move. He can reply by moving his bishop to d3, which gives his king a new flight square (b5) and also makes a threat against White's rook. But now White executes his threats against b8: RxN; and then when Black recaptures RxR, White has BxR.
This last sequence shows that there's more than one way to take advantage of a piece that is subject to a relative pin. You may be able to attack it with a pawn (perhaps with a bit of preparation first); or, as in the second scenario just described, you may be able to make a threat that requires the relatively pinned piece to leave its square and sacrifice whatever lies behind it.