Words fail to describe this, really, you just need to watch. Sit back, turn up the speakers, and watch the whole thing.
The one thing you need to know: this piece is a theme and variations, so it's a musical theme (a melody) that's presented and then Herr Roman Kim plays multiple variations on that theme. You'll still recognize parts of the original theme, but it might be played higher, lower, or with 'extra' notes within the same melody.
Think of it as one car (the theme) with lots of options and it comes in many different colors (the variations)--and they're all on the showroom floor.
P.S. Don't try this at home.