You look at White's capturing options and see that he would like to play QxQ, but to make it rewarding he needs to loosen Black’s queen first. This he tries to do by adding one of his own rooks to Black’s back rank—with check—via Re8. If Black plays RxR, White plays QxQ. Again, though, Black has the option of moving his king to g7; so now what? Here, unlike in the previous position, playing RxR then ruins everything for White: Black replies RxR, and now his queen has just as much protection as it did at the start. But this position also differs from the prior one because Black has a second rook loose on his back rank. So White replies to Kg7 with QxQ. If Black recaptures RxQ, now White has RxRa8. In effect Black’s rook on c8 gets pinned to the other rook when White plays Re8. It's another example of the same point: think broadly about what your rook can do once it's on the back rank.