In Filler, your goal is to fill up 2/3 of the level. You have a limited number of lives and balls per level, but this amount increases with each level.
A note: I guess I'm a lot more casual of a player than most people. I made the game, but I've never made it past level 25... some of the guys on Kong have gotten up into the 70s. The game isn't really optimized for that, so if you make it past 30 or so go ahead and feel like you "beat it." =]
Another note: If you would like to host Filler on your own site, feel free to grab the SWF! It's got a flashVar on it so you can make your own high score table... when you embed the SWF, just use filler.swf?hsname=SOME_ST RING