OAuth is a way for one app to get limited permission to use information or features from another service. For example, a calendar app might ask permission to see your schedule without asking you to give it your email password.
Think of it as giving a visitor a pass for one room instead of handing over the key to your whole house. OAuth is about granting access; by itself, it does not prove who you are. A service that offers “Sign in with…” may use a related system called OpenID Connect to handle sign-in.