White knight is forking the black king and rook. It is particularly effective to fork a king: the rules require immediate attention to a threat to the king. In this situation, Black cannot choose to defend another piece, Black cannot make an intermediate move (zwischenzug) to complicate the situation; the king must be moved, after which White can capture the rook.
In chess, a fork is a tactic that uses one piece to attack two of the opponent´s pieces at the same time, hoping to achieve material advantage because the opponent can only counter one of the two threats.
Knights are often used for forks: they jump to a position from where they attack two pieces.
Pawns can also fork enemy pieces: by moving a pawn forward, it may attack two pieces: one diagonally to the left and one diagonally to the right. In the diagram, the black pawn is forking the two white rooks. (Remember that by convention the board is oriented with Black´s first row at the top, so the black pawn is moving downward.)
A queen move also often attacks two pieces at the same time, but this is only useful if both pieces are undefended, or if one is the opposing king. Since the queen is more valuable than the pieces it is attacking, it is usually only profitable for it to capture undefended pieces.
The term royal fork is sometimes used to describe the situation where the king and queen are forked, and when three or more pieces are attacked by a knight concomitantly the situation is sometimes referred to as a "family check", especially (though not exclusively) if the king is one of the pieces so menaced.
The following example of a fork is from the first round of the FIDE World Chess Championship, 2004 between Mohamed Tissir and Alexey Dreev. After White´s 33rd move the following position was reached:
After 33...Nf2+ 34.Kg1 (the only legal move) 34...Nd3, White resigned. In the final position the black knight is forking the white queen and rook, so that after the queen moves away, White will lose the exchange (a rook for a minor piece).
These moves are given in algebraic chess notation.