The idea is simple, build and maintain your very own space program.
I'm willing to bet you'll start Kerbal Space Program with no knowledge of astrophysics or experience in building your own shuttle, well don't worry about that because you have access to a good number of tutorials to start you off.
Surprisingly building your first rocket isn't that difficult, it doesn't take a genius to slap on a couple of fuel pods, an engine. The real challenge starts when you want to achieve orbit for the first time. Odds are you'll spend your first couple of hours of the game watching your newly created rocket fall from the sky as it runs out of fuel, or spins wildly out of control and crashes back to Kerbin.
The game itself is still in an alpha state, which means the game has a very long way to go before it's considered "finished". There is still plenty of things to do however, there are a decent number of planets and moons for you to visit (or at least attempt), each with their own unique planetary conditions.
One of my favourite things about Kerbal are the mods, something you should definitely consider checking out once you've spent enough time in the game. Kerbal Space Program has a big community of user-shared crafts and mods which is only going to get bigger over time.
Check out KSP on Steam, you won't regret it.
Kerbal Space Program is a fantastic use of your time, you'll spend countless hours constructing, perfecting and maintaining your rockets, satellites, space stations and bases. The learning curve is a little steep but once you get the hang of it you'll be achieving orbit (and more difficult tasks) in no time.