Using Express.js and MongoDB + mongoose, these backend files provide standard funcitonality for a social media platform. In this case, the platform has users post "Thoughts" to which other users can post "Reactions" along with the ability for users to be able to make friends with other users on the site. The included routes allow for CRUD operations on users and thoughts, create/delete friendships and reactions.
This video goes over how to connect your database, how the users, posts, and comments were setup, and how to hit each of the provided routes.