So you're a "bubble tank" in a world of other dangerous "bubbles." Use your barrel and missiles to destroy them. The bubbles that made them up go to you, thus making you stronger. However, if another tank hits you with their missile, you lose a bubble, which makes you weaker. Go to different realms, or bubbles, to challenge other tanks who are also trying to be king of the Bubble world. Dominate and make yourself the most powerful tank ever!
See the BEGINNING of the game, which gives you all the instructions, as well as a brief summary.