When an application is developed, the data is stored on the server side and then sent to the client or user side, this server-side is known as the Backend. The backend is the place where all the internal operations of the applications or websites are carried out and this is where the backend developer works.
A backend developer is responsible for writing and executing the code that makes applications and websites work the way they are supposed to, this code is executed on the web server. In addition to writing the backend, these developers should be responsible for creating the APIs that the website's mobile app will use.