Prepared is a keyword buff: Its completely useless on its own, but has benefits when paired with characters who can utilize the prepared ability within their special abilities. Most of these involve the Solo Gang (Young Han, Young Chewie, L337, Qi Ra and Young Lando. )
Prepared does not always translate to damage bonuses. For example, Young Chewie will revive allies while he is prepared, while lando can grant speed up with prepared. The unique "boost" prepared gives is listed at the bottom of the ability. Not all abilities have prepared "boosts".
Ideally you want to have as many characters with prepared as possible. You can pass prepared around by many ways.