Skewers
A skewer is a pin turned inside out: instead of the LESS valuable piece being in front (forced to shield something behind it), the MORE valuable piece is in front, forced to move out of the attack -- and when it does, the attacker captures the lesser piece revealed behind it. The geometry is identical to a pin (a slider attacking through one piece to another on the same line); what changes is just which piece is worth more.
The most common skewer in actual games is a check: a rook or queen checks the enemy king, the king must move (it has no choice), and whatever piece was standing behind the king on that line is captured next. Because the front piece in a skewer is usually forced to move -- especially when it's the king, and it's check -- skewers tend to win material with total certainty, unlike a pin, where the opponent might just accept the pinned piece is stuck and play around it.
When scanning for skewers, look along every open rank, file, and diagonal your sliding pieces control, and ask: is there an enemy piece there, and does it have something valuable lined up directly behind it?
Worked examples
Skewer (Re1 skewers king in front of rook)
The rook on e1 attacks along the e-file through the king on e4 to the rook behind it on e8 -- the king must move, and the rook falls.
Skewer through a lesser piece behind it
The bishop on b5 attacks the c6 knight, which is worth more than the d7 pawn shielded behind it -- a skewer rather than a pin, since the front piece outvalues the piece behind it.
Test yourself
Find the rook move that skewers the king to the rook behind it.