The previous pins all occurred along diagonals; here is one that runs vertically. When you scan the enemy king's lines you see that Black's queen is an obvious candidate for a pin. White just needs to bring force to bear down the e-file, and this he does with Re1. Again, since the queen is being pinned the piece doing the pinning needs protection, and in this case White’s queen on d1 provides the backup. One way or another Black loses his queen (for a rook) on the next move. Another way to see the point is defensively. You look at the checks Black threatens and see that he is about to mate with Qxh2#. Playing g2-g3 is one way to defend against this—but of course Re1 is better, as it pins and then wins the attacker.