It is protected by an energy shield which is generated from the nearby forest moon of Endor.

6 seconds sound clip from the Star Wars: Episode VI - Return of the Jedi movie soundboard.

You can hear this line at 00:50:03.109 in the Blu-ray version of the movie.

Quote context

[...]

- But most important of all, we've learned that the Emperor himself is personally overseeing the final stages of the construction of this Death Star.

- Many Bothans died to bring us this information.

- Admiral Ackbar, please.

- You can see here the Death Star orbiting the forest moon of Endor.

- Although the weapon systems on this Death Star are 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 shield must be deactivated if any attack is to be attempted.

- Once the shield is down, our cruisers will create a perimeter while the fighters fly into the superstructure, and attempt to knock out the main reactor.

- General Calrissian has volunteered to lead the fighter attack.

- Good luck. You're going to need it.

[...]