From Star Wars: Episode VI - Return of the Jedi
You can see here, the Death Star orbiting the forest moon of Endor. Although the weapon systems on this Death Star or not yet operational, the Death Star does have a strong defense mechanism. It is protected by an energy shield which is generated from the nearby forest moon of Endor.
[the projector shows the Death Star and the yellow lights indicating the shield
The shield must be deactivated, if any attack is to be attempted.
[the projector enlarges the Death Star image
Once the shield is down, our cruisers will create a perimeter, while our fighters fly into the superstructure and attempt to knock out the main reactor.