OAuth
Allow a third-party app to perform actions in a main application on behalf of the user. Like posting comments on Facebook, or publishing a tweet. source
How does Software A give Software B access to User X's data without Software B having access to User X's login credentials. https://stackoverflow.com/a/21175036/4034572
An Illustrated Guide to OAuth and OpenID Connect - https://www.youtube.com/watch?v=t18YB3xDfXI