A delegated user, as laid out in the NIP-26 specification, is a user that is authorized to sign events for a different user. Additional limits can be put on the delegated account, such as time limits.
This may be useful if you wish to keep your main key stored safely offline, but still post as that user with a throwaway key.
The secure account to be delegated.
This is not stored, but only used to sign a token which is stored instead.