Back-end is a part of a web application. It receives data from a user or other systems and prepares a responsive output. Back-end system works mostly with databases and components of data processing. It responds to front-end requests.
A back-end developer typically creates the essence of computational and logic part of a web application. The developer builds services and components which interact with a user indirectly via front-end. A back-end developer usually has strong programming skills in Python, C++, Java and other high-level programming languages.