KapSans it’s basically a super-cheap mini-computer. It comes with no operating system, so you can install anything you want on it.
People turn them into retro consoles, media boxes, game servers, bots, use them to develop and test software, or just use them as a desktop computer for general tasks like email, web surfing, photo management, and more. I’ve even seen videos of people turning them into working, walking, talking robots.
If you are interested in learning more, google it and you’ll find an entire world of people doing really cool things with the Pi.