Black’s rook is suffering from the effects of a pin by White’s queen. One way to ruin a pin is by interposing another piece on the same line―preferably with a check that holds the initiative. This thought leads Black to Qg6+ (his only check anyway). Notice that on top of its other virtues this move also creates the kernel of a discovery; White will have to move his king to h1, and Black then looks for his next check and finds Rf1+, now possible because the rook has been freed from the pin. Of course White can take Black’s rook; aye, the point of Rf1+ is that White must spend a move playing NxR and thus allow Black to play QxQ next move. So a simple check by Black seizes the initiative, dissolves the pin, and finally enables the pinning piece to be taken.