OAuth 2.0 is a widely accepted open standard for authorization that enables a user to give consent for a third-party client application to access protected resources in the name of the user. Giving a third-party client application the right to act in the name of a user, for example, calling an API, is known as authorization delegation.

So, what does this mean?

Let's start by sorting out the concepts used:

Resource owner: The end user.

Client: The third-party client application, for example, a web app or a native mobile app, that wants to call some protected APIs in the name of the end user.

Resource server: The server that exposes the APIs that we want to protect.

Authorization server: The authorization server issues tokens to the client after the resource owner, that is, the end user, has been authenticated. The management of user information and the authentication of users are typically delegated, behind the scenes, to an Identity Provider (IdP)