A discovered attack is a tactical move in which one piece moves away and, in doing so, uncovers an attack by an allied piece that was sitting behind it on the same rank, file, or diagonal.
The mechanism relies on two allied pieces in line: the moving piece (sometimes called the screen piece) steps off its square and clears the path of the discovered piece, which remains stationary and now attacks an enemy target without having moved. What makes this tactic so dangerous is that the opponent faces two simultaneous threats: the moving piece can itself attack or capture, while the discovered piece strikes along the newly opened line.
In practice, scan your position for ranks, files, and diagonals where two of your pieces are aligned with an important enemy piece or square. If the piece blocking the line can move while creating an additional threat of its own, you likely have a discovered attack at your disposal.