A bounty campaign is when a project promises payment for services in their native tokens. These services are typically promotional in some way, such as translating the whitepaper into different languages, writing articles on medium or Steemit, or other tasks to help spread the word. By offering a bounty, the team both spreads the token (and thus the ecosystem), as well as incentivizes people to evangelize the project. After all, by holding the token, the people performing the bounties stand to benefit if the project takes off.

There is no real limit to what can and can't be a bounty. The important part for every team is to make sure that the people who perform the bounties are rewarded, otherwise it's easy to turn a supporter into an enemy.