Web cookies are small data packets sent from a server to a user's browser, stored on the user's device, and used to track activities such as shopping cart contents and user authentication. They enable personalization by remembering user preferences and login details, though they can be vulnerable to hacking. The text provides a tutorial for setting up cookies in a ReactJS application, illustrating the creation of a simple login page where user session data is stored in cookies. It explains how to use the `react-cookie` library for cookie management, allowing the app to conditionally render a welcome page based on the presence of a user cookie. The tutorial emphasizes the importance of secure cookie handling and directs users to GitHub for the complete code, while also suggesting Clerk as a tool for easy authentication integration.