A junk galaxy was a wasted and primarily deserted galaxy used to store refuse from other parts of space. They were usually, but may not always have been, located on the outer fringes of great space-faring empires. The planets within the galaxy were typically covered in garbage and scrap, and junkers and scavengers, as well as the Doctor, sifted through it all to find useful or valuable salvage.
Seffilun 27 and Seffilun 59 were both located within a junk galaxy. The Thirteenth Doctor got them confused at one point, and she and her companions wound up on the medical ship Tsuranga after a sonic mine exploded on Seffilun 27. The Doctor was concerned about leaving her TARDIS behind on the planet, as scavengers frequently visited the location. (TV: The Tsuranga Conundrum)