An x-ray is an uncommon and rarely used chess tactic.It is way too easy to overlook and it is not used much. The only pieces that can perform the x-ray are the long range pieces:the queen, rook and bishop. X-rays happen when it seems like something is safe but it isn't. They are common on the back rank. Here is an example from my games:
It was me playing against a kid about my age. The x-ray tactic allowed me to gain a whole rook. Feel free to add comments and edit!