This is a cargo barge, specifically the kind utilized for logistical support of the original Death Star. Cargo barges represent a class of spacecraft designed for the transportation of substantial quantities of cargo.
A quantity of 13,000 cargo barges functioned as support vessels for the Death Star I, and each of these barges was armed with a pair of twin laser cannons. Among the known types were the Eliall-class cargo barge and the Nyubba-class cargo barge driver. Furthermore, in 3 ABY, cargo barges were present in orbit around Coruscant; it was beneath these barges that members of Rogue Squadron concealed their X-wing starfighters in the time leading up to the Battle over Coruscant.