It is key to getting those big high scores. When you shoot down an alien with a white number superimposed over it, a corresponding symbol (e.g. x3) will appear next to your score at the bottom of the screen. The point value of everything you shoot down will now be multiplied by the symbol. The multiplier effect will also work its magic on your accuracy bonus at the end of each wave. The sad part is that the multiplier is lost after each wave or when the ship is destroyed.