A skewer in chess is a tactical device in which an attacking piece targets a high-value enemy piece, forcing it to move out of the line of attack — and exposing a less valuable piece behind it, which can then be captured.
A skewer is essentially the reverse of a pin: instead of immobilizing a piece because it shields something important behind it, the skewer forces the more valuable piece to flee, abandoning whatever stood in its shadow. A classic example: a rook delivers check to a king that stands in front of an enemy queen on the same file — the king must move, and the rook captures the queen for free.
In practice, skewers are carried out by long-range pieces — the queen, rook, and bishop. Train yourself to spot alignments where a king or queen stands in front of a less valuable piece along the same rank, file, or diagonal. Those are the positions where a skewer becomes possible.