Back-end development
The backend of an application can be seen as the 'brains' of the application. It is where the complex logic of an application lives and where the data is stored inside a database.
The Front-end shows and collects data to/from the end-user. It does that by communicating with the Back-end over an API.
A well-structured backend is generic and can 'serve' multiple front-ends or can interact with the back-ends of other applications to create an integrated service for the end-user.
Not every back-end is a CMS system, but a CMS system is always part of a back-end.
A CMS, or Content Management System, gives specific users/admins the tools to manage and configure the data & logic that is available in the back-end.
Through a CMS system, people with limited technical knowledge can still manage and configure complex tasks and data in a digital application.