The backend is the behind-the-scenes part of an app that handles work such as storing information and responding to requests. It is like the kitchen that prepares an order after someone chooses a meal from the menu.
When a visitor signs in, the backend may check the account details and send back a result for the website to show. The visitor usually interacts with the frontend, while the backend handles parts they do not see directly.
Backend work often involves data and services, so it needs careful checks to protect information and handle errors.