Galaga is an arcade game where the player controls a spaceship at the bottom of the screen.
At the start of each stage, the area is empty, but as the game goes on, enemy aliens fly in groups. When all of the enemies are on screen, they will come down on the player's ship in groups of one or more and may shoot it or crash into it. The player can shoot at the enemies at any time during the stage.
When all of the enemies are dead, the player moves on to the next stage.
Have fun and good luck!