Edit: I should have added my own explanation as it's not as obvious to everyone else what an NFT is. If we continue the analogy with paper, then a NFT is a piece of paper with a unique identifier that cannot be copied. If you have that piece of paper you know that A) No one can have the same piece of paper without having that actual paper and B) no one can fool someone else they have that same paper (based on the ID). NFTs have nothing to do with URLs (although some NFTs do have URLs inside of them), the main point is to have something with a unique ID that is unique within that specific blockchain.