Kessel first appeared in dialogue in A New Hope, where subjects such as its spice mines and the Kessel Run were mentioned in passing. The Kessel Run has been the topic of much speculation given Solo's confusing boast of completing the Kessel Run in "less than 12 parsecs." Given that a parsec is a unit of measurement and not of time, it hardly seemed like a speed record.
The Star Wars Technical Journal came up with a workable explanation that saw a Kessel Runner have to deposit smuggled spice shipments in between freighters traveling at a fixed speed, so that the shorter distance covered by said freighters during a Run was worth bragging about. Another version of the Kessel Run, spun out of the Jedi Academy Trilogy, required a smuggling ship to skirt close to the black holes of the Maw Cluster, shortening the distance only through skillful piloting and powerful engines.
Of course, the simplest solution is the one favored by George Lucas, and the one that appeared in the screenplay for A New Hope. Han's boast was nothing more than a lie, meant to hoodwink provincial customers. Obi-Wan's knowing glance suggests he saw right through Solo's meaningless bragging.