Your goal in the game of Filler is to fill 2/3 of the screen with filler balls while avoiding the small bouncing balls. You click and hold the mouse button to create the filler balls. The longer you hold the mouse button the bigger the filler ball you create. You get bonus points for using fewer balls, having time left on the timer, filling more of the screen, and the number of lives remaining.
The first few levels are easy, but every level adds a new bouncing ball and makes things much more difficult. You need to learn a few tricks to restrict or control the movement of the bouncing balls if you are going to survive as the levels progress.