In chess, a skewer is a situation where a two pieces are attacked along the same line, where one piece is in front, protecting the other, similar to pinning. The difference between the two, however, is that in a pin, the less valuable piece is protecting the more valuable, while in a skewer, it is the other way around: the more valuable piece is in front of the less valuable one.
For example: a queen on b4 is threatened by a bishop on d2. Behind the queen, on square a5, is a rook of the same color. If the queen is moved, the other player can take the rook behind it with their bishop. If the queen does not or cannot take the bishop, then the next-best thing would be to move the queen out of the bishop's path, allowing it to take the rook, a less valuable piece than the queen.
An absolute skewer is one where the king is being threatened, and the other, less valuable piece is behind it. Since the rules of chess require the king to get out of check somehow, if the threatening piece cannot be taken or blocked off, the king must move, leaving the opponent's piece to take the one that was behind the king.
Since it puts the opponent's most valuable piece in direct threat, a skewer is generally more effective than a pin, though it is seen less often in actual play. The player being "skewered" often cannot avoid losing a piece. However, if that player can move the piece in front to a position where it causes check, it will allow for another turn to move the other piece out of danger. A skewer is also called a "thrust."