It's a number in a database that's assigned an "owner". Some NFTs can be controlled by a central authority, kind of like your bank account, and some of them are

It's a number in a database that's assigned an "owner". Some NFTs can be controlled by a central authority, kind of like your bank account, and some of them are totally decentralized and can only be modified/traded if you, the owner, wills it so. Someone can associate some data with that unique number, if they want.