The Kessel Run was an 18-parsec (Roughly 58 Light Years - Wikipedia) route used by smugglers to move glitterstim spice from Kessel to an area south of the Si'Klaata Cluster without getting caught by the Imperial ships that were guarding the movement of spice from Kessel's mines. Worlds along the Kessel Run included Fwillsving, Randa, Rion, and possibly Zerm.
It took travelers in real space around The Maw (Black hole cluster) leading them to an uninhabitable—but far easier to navigate—area of space called The Pit, which was an asteroid cluster encased in a nebula arm making sensors as well as pilots go virtually blind. Thus there was a high chance that pilots, weary from the long flight through real space, would crash into an asteroid.
Han Solo claimed that his Millennium Falcon "made the Kessel Run in less than twelve parsecs". A parsec is a unit of distance, not time. Solo was not referring directly to his ship's speed when he made this claim. Instead, he was referring to the shorter route he was able to travel by skirting the nearby Maw black hole cluster, thus making the run in under the standard distance, he may have indirectly referred to the speed of his ship here because to be able to go closer to black hole and still be able to get out of its gravitational pull you will need to be able to go faster.