A bounty is a way to get additional attention for a question by offering some of your own reputation for great answers.
The bounty system is a way for users with over 75 reputation points to provide a 50-500 reputation points incentive to answer a question that's at least 2 days old.
Bounties expire after seven days. After a bounty expires, there is an additional 24 hour window for the bounty to be awarded so that late answers may be considered. If it remains unawarded and one or more answers posted during the bounty period have a score of 2 or more, half the bounty will be automatically awarded to the one with the highest score.