A loose piece makes a great target anytime. It therefore makes a fine basis for a relative pin: if you attack two enemy pieces in a line and the rear piece is loose, it becomes costly for your opponent to move the piece in front. This simple point forms the basis of a common and valuable pattern. Suppose one of your pieces is aimed at an enemy piece; for the sake of the example, assume your rook is aimed at your opponent’s bishop. On the other side of the bishop is your opponent’s rook. Both his bishop and his rook are protected. This might not look like a pin, or indeed like anything; your rook simply is aimed at a piece that it can't afford to take, and behind it is another piece you can't afford to take. Ah, but if you somehow can loosen your adversary's rook, the bishop in front of it suddenly does become pinned; for behind the bishop now lies a target that now is vulnerable. Now the bishop can be attacked with a pawn and may not be able to afford to run away (or if it does move, you can take the rook behind it); or perhaps something the bishop used to protect now is loose, since the bishop no longer is free to leave its square. This is the most interesting and important technique to understand involving the creation of relative pins: the art of loosening a screened piece, so that the enemy piece in front of it becomes pinned.
Let's apply this thinking to the current example. What does White now attack? There are lots of ways that asking this question during your games can lead to good ideas. Here is one: you may find an enemy piece you attack that has another enemy piece behind it. In this case we find that pattern on the c-file, where White’s rook bears down on a bishop and rook, one behind the other. At first it doesn't look like a pin or anything else useful for White: the bishop on c7 is guarded by the rook on c8, which in turn is guarded by the rook next door. So what?
Well, if you could loosen the c8 rook the bishop would be pinned, because by moving it would expose the rook in the rear to uncompensated capture; and if the bishop were pinned in this way your pawn on b5 would assume a new significance as an attacker. So think about loosening the rook on c8. White has nothing he can use to capture it and force Black to replace it, but there are other ways to loosen a piece: one can pry away its defender, perhaps by distracting it—i.e., by attacking something else the defender protects. Here that means getting the d8 rook off the back rank by going after the other piece it guards—the bishop on d7, which White can take with NxB. Black replies RxN. Now White has succeeded in splitting Black’s rooks, depriving them of the protection they offered to each other. Since the rook on c8 is loose, Black can’t move his bishop without suffering RxR uncompensated. In other words, the bishop is pinned.
What to do with a pinned bishop? Idea one is to throw a pawn at it, as with b5-b6. But as usual you consider whether the square where you propose to move your attacking piece is safe. It isn’t: Black would be able to take the pawn with his knight. Yet this, too, can be corrected with an exchange. White takes out the knight with BxN, and after the forced recapture e6xB the board is ready for White to play b5-b6. The bishop comes off next move.