Using Auth0 in your applications means that you will be "outsourcing" the authentication process to a centralized login page in the same way that Gmail, YouTube, and any other Google property redirects to accounts.google.com whenever a user signs in.
Your user will authenticate, and Auth0 will generate an ID Token that will be passed back to your application.
In a regular web application:
For security in server-side web apps, Auth0 uses the Authorization Code Flow.
Step-by-step instructions for tasks