A bounty (懸賞金 Kenshōkin, literally meaning "prize money") is a government-issued reward placed on an individual and awarded to whoever can capture or kill that person.
Normally, a bounty is issued by a government official or by the police. The idea is to tempt others to aid with the capture of the criminal so they can be brought to justice quickly. Bounty hunters make careers trying to find and capture these wanted people. Bounties are issued with a "Dead or Alive" disclaimer, meaning the reward will be paid so long as the threat gets eliminated, but bounties are usually only paid in full if the criminal is captured alive; up to 30% of the bounty money may be lost by bringing in someone dead, since no public execution can be held.
The amount of a bounty is determined in response to the perceived threat level of the criminal in question; the greater the threat to the world, the greater the bounty. In the eyes of a pirate, high bounties are thus usually seen as a sign of strength. After all, having been assigned a bounty means that the World Government as well as the Marines acknowledge the threat of the person to whom it is assigned. A bounty in excess of 100,000,000 indicates that the person to whom it was assigned has committed unlawful acts of catastrophic degrees.