You fend them off by shooting them (which might be why they are trying to kill you; who's to say?). As you progress, you can get different power-ups like Triple Shot, which gives you a three-shot spread, or Piercing Shot, which slows down your enemies and more.

You can maneuver around, but depending the color, the circles will start to close in on your location. Some circles will break into other circles as you pop them, some send out a devastating shockwave harming circles or ships (i.e. you) near them. If they catch up with you, it drains your health bar and you can die.

Basically, you try to survive for as long as you can. I honestly don't know if there's an endgame, since I can't live long enough. But it's fun to try. So much fun. Play for free at www.apphut.net.