NFTs are tokens that are individually unique. Each NFT has different properties (non-fungible) and is provably scarce. This is different from tokens such as ERC-20s where every token in a set is identical and has the same properties ('fungible'). You don't care which specific dollar bill you have in your wallet, because they are all identical and worth the same. However, you do care which specific NFT you own, because they all have individual properties that distinguish them from others ('non-fungible').