Also known as the server-side, a backend is the behind-the-scene of web development that ensures everything on the client-side (frontend) works properly.
Frontend and backend work with each other by sending and receiving information to be displayed on a web page for users. For example, when you do any interaction on a website, such as purchasing or loading a web address, your browser sends a request to the server. The server will then process the request and then send back information in the form of frontend code to the browser to be interpreted and displayed.
Some other examples of backend processes are encrypting and decrypting data, running a script for HTML, accessing data, file uploads, and storing records in a database.