Backend, also known as server-side, refers to the part of a software application or website that is not visible to the user, but provides the functionality and data management necessary for the application to work properly. It involves the use of programming languages such as Java, Python, or Ruby to develop and manage the server-side logic, database management and other components that support the frontend. The separation of backend and frontend allows for more efficient development and maintenance, and enables developers with different skill sets to specialise in each area.