Front End

"Front end" is just another term for anything that happens in the browser (also known as the client). We could say that JavaScript, which executes in the browser is a "front end language". When a programming language executes on the server, we could also say it's a "back end language".

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