The idea of the game is to accurately position the missing letter from a varying sized, type-oriented word as it falls down the screen, just as Tetris works. It is great to see games made by graphic designers for graphic designers. KERN is, in fact, one of a series of design-oriented applications made by US design studio FORMation for the iPhone. I have not purchased this game yet because I am cheap and seem to stray away from paying for applications. I also do not have the time to play them, but it seems lik a lot of fun, so I'll give it a recommendation.
Below are a few snapshots of the game, so you can get a feel for the gameplay and interface.