You know the drill: examine checks and captures in search of any sequences that will leave enemy pieces lined up for forks, pins, or (our concern here) skewers. White has no useful checks, but he has a capture that is of some interest: BxN. It provokes the recapture BxB, a classic trade of minor pieces. Now consider the board as it then would look. The exchange would have left Black’s bishops lined up on the f-file—and both of them loose. Two loose pieces make a great target for a skewer, so White plays Rf2 and wins one of them next move. You might also have seen the idea by noticing Black’s knight and bishop on the f-file from the start. They don’t make for a good skewer because the knight has protection and so cannot be chased out of the way. But one way to deal with a piece that has protection is to capture it; for after the recapture the piece that replaces it may be loose. That is White’s procedure here.