Back End

"Back end" is just another term for the server. When a programming language executes on the server, we could also say it's a "back end language". This is in contrast to the "front end" being the browser (the client) and we would say that JavaScript that executes in the browser is a front end language.

When a programmer says they're a "back-end programmer", it means they focus most of their job effort on things that happen on the server. If someone works in the back end and the front end, they might call themselves a "full stack developer".