Presumably, the player was a tailgunner of a spaceship being chased by
enemies. By moving crosshairs with the joystick, the player's goal was
to shoot the chasing ships before they overtook the player's ship. The
game ended after 3 or so ships passed you.
In addition to the gun, the player had a shield button as a last resort
to stop the enemy ships. Once they hit the shield, the targets would be
stunned for a few seconds. Shield use was limited.