Fork

A double attack is a move that creates two threats at once. A fork is one specific case: one piece attacks two or more targets. Because the opponent gets only one move per turn, saving one of those targets often leaves another available to capture.

Why a fork can win pieces

For example, a knight may give check, meaning that it directly attacks the king, while also attacking the queen. The opponent must answer the check first. If that reply cannot also save the queen, the knight may capture it next. Material means each side's pieces and pawns; winning material means gaining something more valuable than what was given up.

Why it may not work

Every piece, including a pawn or king, can make a fork. Knights do it often because they jump over other pieces and their attacks cannot be blocked. Even so, a fork does not guarantee a gain: the forking piece may be captured, one defense may save both targets, or the expected capture may cost more than it gains.