The backend of an application is any part of the application that the user doesn't directly interact with. Generally our backend contains an API and a database. The user interacts with the frontend, which in turn makes a request to the API, which will then query the database and return results.
MongoDB Query and Update Operators
MongoDB Projection (and operators)
how to use express - what is app? middleware? what is express good for?
debugging your api (status codes, mongo errors in the console, etc.)