Master this and many more motifs
Only bishops, rooks, and queens can make a skewer because these pieces have a long-range attack.
Sometimes a skewer can win material immediately. All puzzles in this level are cases where you can win material with a skewer.
White can win material with the skewer Rf8+.
Black cannot block the check, and has to move with the king out of the check. This reveals the rook on a8 behind the king, which White can capture with Rxa8.
Black can regain material with the skewer Be1.
Black attacks the rook on d2, and the unproctected knight on c3 is behind it.
White cannot move the rook to a square where it protects the knight, as both c2 and d3 are guarded by the queen on f5.
Whenever you make an attack with a long-ranged piece, also consider what is behind the attacked piece, to quickly find any pins and skewers.
An attack on a piece is called a pin, when moving it out of the attack would expose a more valuable piece behind it.
The difference between a pin and a skewer is mostly in how the opponent can react to the attack. With a pin, the opponent does not want to, or is not able to move the attacked piece, whereas with a pin the opponent prefers to move it.