The guide provides a detailed walkthrough on setting up authentication using PropelAuth for a product built with FastAPI and React, specifically employing Next.js and TypeScript for the frontend. PropelAuth simplifies the authentication process by providing ready-to-use UIs for user signup, login, and account management, which can be customized via its dashboard. The guide covers the installation and configuration of PropelAuth in a Next.js application, including setting up the AuthProvider to manage user information and display user data using React hooks. It explains how to handle authentication in HTTP requests from the frontend to a FastAPI backend, demonstrating how to secure API endpoints using PropelAuth to ensure that only authenticated users can access certain routes. Additionally, the guide touches on advanced features such as social logins, passwordless login options, and organization management, encouraging further enhancement of the authentication system.