first of all i love star wars its my second favourite sci-fi series of all time. but a parsec is a measure of distance not time. damn you george lucas and your half baked research
GEEK ALERT!!!!
BUT - the Kessel run was the trip to Kessel (a prison planet that happened to produce a valuable drug (using convict labour) set in close proximity to the Maw - a group of 9 (IIRC) black holes - so to get to it quickest depended on a pilot's skills in balancing the influence of those black holes to skirt close to them & take the shortest course - hence the 12 Parsecs claim.
Most worrying bit is that came from memory, not google - I wonder what useful information could have used that space......
I'm now going to google to check the Parsecs explanation (which came from the extended universe novels, so after Lucas made the original mistake).