A Regional Naval Supply Area, or RNSA, was a supply and storage facilities for arms, ammunition, and other supplies. Security was usually extremely tight with the highest enforcement in clearance.
The RNSA at Gall was infiltrated and a load of explosives went triggered and sent out with the hopes for reaching the Death Star. As the Undauntable needed ammunition before the Death Star, the containers were shipped there instead, and the ship exploded.
Daala and Wilhuff Tarkin focused on the RNSA at Gall to find any Rebel spies that might have been hiding in the Imperial Navy.