Users with existing Galaxy user accounts (e.g., via Galaxy username and password) can associate their account with their 3rd party identities. For instance, if a user associates their Galaxy account with their Google account, then they can log in to Galaxy either using their Galaxy username and password, or their Google account. Whichever method they use they will be assuming same Galaxy user account, hence having access to the same histories, workflows, datasets, libraries, etc.
See more information, including a list of supported identity providers, here.
This is your only defined external identity. If you delete this identity, you will be logged out. To log back in you will need to use a password associated with your account, or reconnect to this third party identity. If you don't know your Galaxy user password, you can reset it or contact an administrator for help.