Back-end developers are responsible for the "back-end" of a website, application, or program. Where a front-end developer deals with the visible components of an application, the work of a back-end developer is not visible to the user. You can therefore also say that the back-end developer works under the hood.
The back-end often consists of API’s that allow communication with other programs. Therefore, various functionalities can be implemented. Think for example of the implementation of payment systems. There are many back-end coding languages. Frequently used languages are PHP, C++, Java, Swift and Python.