The situation is tense for White; notice that his queen is pinned. He could try moving his king, but one wants to avoid playing defensive moves too quickly. Better to first consider every offensive option and its results. Here that means not just looking for pins like the inadequate Re3, but considering every available check and Black’s possible replies. Since White’s queen is pinned his only check is Rh7+. The d-file is off-limits to Black’s king because White’s queen is there, and the same goes for e6. Thus the king’s only flight square is e8. But this lands the king on the same light-squared diagonal as Black’s queen, so you immediately look for a pin with your light-squared bishop. Bb5 does the trick. Now Black can’t play QxQ because his queen is paralyzed. The best he can do is QxB, allowing White to play QxQ and effectively end the game.