A Skewer is like a "Reverse Pin." In a pin, the valuable piece is at the back. In a skewer, the most valuable piece is at the front!
By attacking a valuable piece (like the King), you force it to move away. This leaves the piece sitting behind it completely unprotected.
Skewers are common in endgames, especially with Rooks and Bishops. Because they act across long distances, your opponent might not realize their King and a Rook are lined up until it's too late.
Strategically, a skewer is a powerful way to win material. Even if the piece in front is defended, it almost always has to move because its value is so high. It’s essentially a "look through" attack—you look through the first piece to see your true target behind it.
When defending, try to avoid placing your heavy pieces (Queen and Rooks) on the same line as your King. Staying off the same diagonal or rank makes it much harder for your opponent to land a successful skewer.