It's a spaceflight simulator. You get to design your own spacecraft (only rockets in the free version, but spaceplanes have been added in more recent, but no longer free, versions). Then you get to fly it.
It's very hard.
You can get the free version, which I am playing right now, or you can pay $15 USD to pre-order and get all of the updates for free.
The game has support for modding, and there's a reasonably large community of KSP modders.
Seriously, go to the website, download the free version, follow the installation instructions, and try it. Your first rocket WILL crash, no exceptions, but keep trying and eventually you'll end up in orbit around entirely the wrong object like me due to a hilariously overpowered rocket.