This is one tricky game! Don't be surprised if you don't last more than a few seconds. Ultimately the aim is to leap from platform to platform, for as long as you can. You can move left and right with the arrow keys, and up will make you jump (or double-jump). However, the longer you remain on a platform, the faster it vanishes. And the more you jump, the smaller you get! Hit the top or the bottom of the screen, or anything red, and it's game over too.
Good luck :)