[ClearCase_guy]

There are some people who rationalize and explain away Han Solo's statement -- so it not definitively a "goof". But the general interpretation is that the Kessel Run (which is never characterized) is probably a journey from point A to point B. A skillful (fast) pilot might be expected to make the trip in an especially short period of time. Solo uses a unit of distance (parsec) to boast of his prowess in getting from point A to point B.

The general feeling is that "parsec" sounded like a unit of time, so George Lucas used it to show that Han Solo's ship was very fast.

[Mr. Silverback]

Actually, they retconned that:

http://starwars.wikia.com/wiki/Kessel_run

[Romulus]

Moreover, parsec is an Earth-defined term because it derives from the size of our planet’s orbit. A parsec on a fictitious Star Wars planet — especially one with two suns, as I recall — is likely to be almost anything.

[MeanWestTexan]

See, I always took that to mean that he was able to go from point A to point B using a dangerous shortcut because his ship was so good.