[Darth Marsden]

They discuss that in Darths and Droids. Give me a minute and I'll find the exact one.

EDIT: Got it.

Originally Posted by Darths and Droids

Many people have considered the idea of bringing "Balance to the Force" as silly, because that (they assume) would mean balancing the Light Side and the Dark Side. In other words, increasing the amount of evil in the Galaxy, since during The Phantom Menace it is mostly still good. Obviously, if this was what was intended by this phrase, that's just stupid.

According to George Lucas, in a documentary released before The Phantom Menace was made:

"Anakin Skywalker, a young boy who is destined to be a significant player in bringing balance back to the Force and to the Republic. [...] Which brings us up to the films 4, 5, and 6, in which Anakin's offspring redeem him and allow him to fulfill the prophecy where he brings Balance to the Force by doing away with the Sith and getting rid of evil in the universe."

So it is meant to be the existence of evil that is the imbalance, and restoring balance involves defeating and eliminating the evil. Which makes a whole lot more sense. Unfortunately the choice of phrasing turned out to be ambiguous and easily misinterpreted.