Grab the fireball with your mouse, then drag and throw it to light torches on each level. Light the required number of torches (this may not always be 100% of them), to procede to the next level.

Press 'S' to suicide if you dont think the ball will his any more torches, or if you'd like to speed up the end of a level where crates are still burning (levels will not end until all crates have finished burning, in case they might light another torch).

Lighting all of the torches with one toss is called an 'Ace', and the bonus levels are unlocked by acing a certain number of levels.