The planet known as Kessel was first mentioned by both C-3PO and Han Solo in A New Hope but never featured in a film before. C-3PO talks about being sent to the mines of Kessel. Han Solo said his ship, the Millennium Falcon could do the Kessel Run in 12 Parsecs.
A parsec is a unit of distance, not as originally implied, a unit of time. The producers have managed to turn the whole thing round and made Parsec be used in the right sense of the word. The rocky planet Kessel is located inside a giant nebula know as the Maw Nebula that is light years across.
The quickest known and safest route in and out to Kessel is through the Run which is 20 light years. Han is able to plot a course through the nebula which would only take 12 light years to get out. The nebula contains a variety of asteroids, living creatures and a miniature black hole, all of which Han Solo expertly navigates through.
The planet is controlled by the Pyke Syndicate criminal organisation, it has an uneasy truce with the Crimson Dawn that is led by Darth Maul and Dryden Vos. Any involvement of Crimson Dawn would spark an all out war between the two syndicates so the Crimson Dawn couldn't get to the planet. A group that is unknown to the Pyke Syndicate could get in and out with some Coaxium ore.
Kessel is one of the few places that isn't controlled by the Empire so is a perfect location to steal Coaxium from. The atmosphere of the planet is noxious and hard to breathe and slaves and criminals are used to do the mining. Han Solo pretends to be a slave given to the Pyke Syndicate by Qi'ra but its just a ruse for Han Solo to get into the Coaxium store room and still some ore.
On the outskirts of the nebula is the desert/water world of Savareen. Savareen is used by smugglers who have Coaxium to visit to refine the coaxium into a safe, reliable energy form. As well as Coaxium being a highly valued mineral, it is a high combustible material which makes Savareen an important location to visit after exiting the Kessel run.