There's a free demo available on the site, no strings attached.
Essentially, it's a game about building rockets.
Building rockets, and getting to space.
Or building horrible deathtraps and watching their faces as it all goes to hell.
Or building planes, and flying around on the fully simulated entire planet you have to work with.
Basically imagine a flight simulator in which you build the air/spacecraft using a highly intuitive ship creation system.
I have wasted countless hours on this game, it's freaking amazing.
And to this day, Jebediah still does not give one pineapple in the face of death.
Also, you have an entire solar system to explore, but good luck managing a landing, it's a lot harder than it seems, and I've only managed to crash into the moon, but SOME DAY, I SHALL ACTUALLY MANAGE A LUNAR LANDING