Skewer - is an attack on 2 pieces that both stand in the line of attack. After the first piece moves out of the way  a piece behind it is lost.
The idea can be described as the "reverse pin". Here a more valuable piece is attacked first, while the less important piece is behind it.
(Unlike pin where more important piece is hidden behind a less valuable piece)

One of the most common examples of the tactic occurs in Rook Endgames:

White attempts to promote their Pawn. However Black Rook prevents it. It seems that on any move from the White Rook their very important passed Pawn is lost. 1) Ra8-h8! Ra2xa7 2)Rh8-h7+ (skewer) and 3)Rh7xa7