NFT stands for nun-fungible token. It’s something that is unique and can’t be replaced with anything else.
A bitcoin, for example, is fungible as you can trade them for other bitcoin. A one-of-a-kind, limited-edition baseball card, for example, is unique and can’t be replaced.
An original piece of art is also non-fungible. You can copy it but only one person will have the original piece.
While most NFTs are currently part of the Ethereum blockchain, any blockchain can implement its own version of NFTs. So, in the future, NFTs will be an integral part of blockchains, but different than cryptocurrency.