ERC-721 introduced a standard for NFTs (Non-fungible tokens), akin to building codes or community standardized rules.
In gist, ERC-721 established a standard token interface for the creation of unique digital tokens on Ethereum. (aka deeds)
The main aspects of the standard are simple, yet important.
This functionality seems very basic. However, it enables the transferring of tokens from account to account, retrieval of token balances, tracking ownership all the way to specific IDs, as well as keeping track of the total number of tokens.
ERC-721 allows users of Ethereum to issue unique assets such as paintings, collectibles and memberships as tokens.
NFTs allow for anyone to create and maintain an authentic path of provenance.
Below are some more resources for learning about Ethereum standards.