Remember the old fifties sci-fi movies and book jackets, where the rocket would land on the Moon (or Mars or Venus) on it's tail, balancing nicely on it's "fins"? No? Well, the photo below should refresh your memory.
Anyway, SpaceX is trying to do the same kind of thing with it's reusable Falcon 9 booster rocket, as part of their contract with NASA to resupply the International Space Station. After powering it's payload into orbit, the Falcon 9 is supposed to return to a floating landing pad and touch down on it's rump. Except it didn't. It's a tough thing to do but if they can perfect it, it will be pretty darned cool.
Before Saturday's launch, SpaceX had put the odds of a successful landing at 50 percent "at best" and likened hitting the bull's eye to "trying to balance a rubber broomstick on your hand in the middle of a wind storm."