Master this and many more motifs
An x-ray defense is a move that protects your piece "through" the attacking piece.
This is much rarer than a normal protection, but sometimes it is the only possible defense.
White's queen on b7 attacks Black's knight on a6.
Running away with the knight to b4 or b8 would leave the bishop on c7 unprotected and allow Qxc7.
Protecting the knight with the rook on a8, by moving Nc8, would leave the rook unprotected and allow Qxa8.
1...Qc8 is the only good move. It x-ray-defends the knight on a6, because Qxa6 can now be answered with Qxa6.
An x-ray defense is sometimes used to defend against a fork.
Here black’s queen on d4 is attacking both of White's rooks (a fork), and 1.Qe5 is the only move to defend both.
While this doesn’t happen so often, it can be easy to overlook, so practicing this skill will be very useful.