In Salesforce, a delegated user is an user that has been given specific permissions to access and manage another user's data. This is typically used in situations where one user needs help managing their data, but doesn't want to give full access to another user.
Delegated users have limited access to an org, and can only perform certain actions on behalf of the other user. For example, a delegated user might be able to view records, create new records, or edit existing records on behalf of the other user. But they wouldn't be able to delete any records or change any org-wide settings.
If you're a delegated user, you'll see (delegated) next to your name in the top right corner of Salesforce. And when you try to perform an action that you don't have permission for, you'll see a message saying that you don't have enough permission.
If you're looking to become a delegated user, you'll need to ask the other person if they're willing to delegate their data to you. Once they've agreed, they can add you as a delegated user in their personal settings page.