What is a Token ID?

All NFTs are identified by a unique unsigned 256 bit number/integer(uint256). This number is stored as a uint256 inside the ERC721 smart contract for the NFT. This number cannot change for the life of the contract. The TokenID+contract address is a globally unique number that identifies this specific digital asset(kind of like a serial number but better). By using uint256 as the data type for this parameter it allows applications to create a universal unique ID(UUID) and can combine with a sha3 hash which are directly convertible to uint256.