The skewer is like a pin, but the other way around. That is, you attack a valuable enemy piece which once moved allows another lesser valued piece be taken.
Or put by professionals:
"The Skewer is a piercing attack which menaces two hostile pieces placed on the same line. As the piece directly attacked moves away, the piece behind it is transfixed on the skewer".Chernev and Reinfeld
In the diagram below we see that the black king is in check must move allowing his queen to be taken.
In the next position the queen is attacked and unless black wants to lose his most powerful piece must be moved. Once the queen is moved the black knight can be captured.
Note: if the black queen could move to a square where she defended the knight the skewer would be pointless.
In the next example the black bishop skewers one rook to another. True the rook can move to protect its partner, but as a bishop (worth 3) is a lower value than the rook (worth 5) there is no way white can escape the loss of material.
Note: When rooks line up on a diagonal like this they are easy prey for a bishop!
The final diagram is from Trotter, Wilson Durham Congress 2006. Black has ben advancing his passed a-pawn and white desperate to stop the pawn plays 53.Ke2. Black follows with 53...Rh1
Did you spot that rook takes pawn allows Black 54... Rh2+ and a skewer on King and rook.
Note: This is an excellent promotion trick, remember it and use it.
Skewers can be deadly!!