EIP-712 is a standard for encoding and signing structured data. The resulting signatures are each tied to a domain that must match with a contract's domain to be considered valid by it. Domains are not guaranteed to be unique. However, if a contract defines its domain properly, wallets can make use of the information to warn users of the potential consequences of a given signature.
Applications that ask users to produce EIP-712 signatures need the full domain as recognized by the contract that will be consuming it. Unless the application only deals with known contracts and known domains, it needs a way to retrieve the domain for a contract. There is no mechanism defined in EIP-712 for this retrieval, so ERC-5267 was developed to fill in that gap.
This website and the eip712domains JavaScript library can help to retrieve the EIP-712 domain of an unknown contract. The domains of ERC-5267 contracts will be reliably recovered, whereas the domains for other contracts are recovered in a best-effort manner, some of it from a list of known domains and others by guessing.