A skewer is similar to a pin except in this case the piece of greater value (for example, the King, Queen or Rook) is in front. An enemy piece attacks the piece and when this piece moves away, it leaves a piece of lesser value exposed to capture.
In the diagram, the White Bishop is threatening to capture the Black Queen. When the Queen moves out of the way, the Bishop captures the Rook.
Below the Bishop is Skewering the Black King and Queen. The King must move out of check and the White Bishop captures the Black Queen!
Can you see how White can win material by using a skewer?
White plays Rh7+ and when the King moves the Rook captures the Queen!