When you open an app or visit a website, you only see the surface. The buttons, the menus, the design – that's called the front end. But behind every click and every page load, there's another layer at work. That layer is the backend (sometimes written back-end or back end).
The backend is where all the important things happen. It stores data, processes requests, and keeps everything running in the background. Without it, even the best-looking site would be useless.
The simplest way to explain backend is this: it's everything you don't see, but that makes your website or app work.