X-ray tactics exploit the power of long-range pieces (bishops, rooks, and queens) to influence squares beyond pieces that stand in their path. While a piece blocks the direct line of attack, the x-ray principle means the long-range piece still exerts latent pressure that can be activated if the blocking piece moves.
A common x-ray situation occurs when two rooks face each other on an open file with only one piece between them. The rook "behind" the obstruction can still influence the squares beyond, meaning if the blocking piece moves or is captured, the x-ray comes to life. Similarly, a queen can x-ray defend a piece through an intervening friendly piece.
X-ray motifs appear frequently in rook and queen endgames. Understanding x-ray influence helps with both offensive and defensive calculations, as pieces that appear blocked may actually control critical squares through the pieces in front of them.