The concept is simple and tetris-like. Take aim with the left and right arrows, shoot with the the up arrow. Put three or more balls of the same color together, and they all fall down. When all the balls have fallen, proceed to the next level!
In the two player version of the game, you also need the J and L keys (for left and right) and K (for shoot).
Frozen-Bubble is widely rumored to be responsible for delaying the Woody release of Debian, the Linux distribution on which Knoppix and Sn°frix are based---so many of the developers were busy playing . . .