"If you say in the first chapter that there is a rifle hanging on the wall, in the second or third chapter it absolutely must go off. If it's not going to be fired, it shouldn't be hanging there."
— Playwright Anton Chekhov (From S. Shchukin, Memoirs. 1911.)
Often confused with foreshadowing (which can be about practically anything) Checkov's Gun refers to specific items to which you call attention while writing. You can leave them alone for a bit, but be darn sure you use that element while it is at least a dim memory in the reader's mind.
In fantasy, this is the amulet that grants a power, the hithero useless sidekick that does something brilliant, etc.