A skewer is the opposite of a pin. The difference between a pin and skewer is that - in a skewer, an attack is created upon two pieces in the same line except that - the more valuable piece is in the front, and behind is a piece of lesser value. The opponent is forced to save the more valuable piece since it's under direct threat, but the end result is that the piece of lesser value behind usually gets captured, giving the attacker a material advantage.