In a game of chess, the term "Fork" is used when a piece attacks 2 or more of the opponents pieces at the same time.
Quite often it's a Pawn or more frequently, a Knight that is responsible for executing a Fork.
White moved his Knight to square e6 placing black's King in check. Much to black's chagrin, his Queen was on f5 which also was a square the Knight was attacking when the check was placed. Thus creating a "Fork".
Because a Knight check can't be blocked, black had to move his King to get out of check, at which point the Knight captured black's Queen.
Example 2: White pushed his pawn to square e4 and as a result, attacked the black's Bishop on square d5 and Rook on square f5 simultaneously.