The main shield is constantly rotating around the spaceship in order to protect it. The purpose of the rotating shield into and thus damaging the spaceship. You can touch the screen to move the main shield. If the approaching meteor is at very high speed, touch and hold the screen to speed up the shield. You will have extra shields throughout the game. You will lose your extra shields as meteors hit them and your main shields will become defenceless. Not to worry as you will get extra shields with every new level passed.