Fork
A fork is when one of your pieces attacks two (or more) of your opponent's pieces at the same time. They can only save one, so you win the other. Knights are famous for forking, but any piece can do it.
White has just played Nc7+: the knight attacks the black king and the rook on a8 at the same time. Black must move the king, and White wins the rook — a royal fork.
Source for further study: Bain, Chess Tactics for Students, ch. 1.