A bounty is an incentive, and they're often used as part of a marketing strategy to incentivize the promotion of initial coin offerings for new cryptos.
Bounty programs have been helping cryptocurrencies get off the ground for quite some time. Any time a new product comes onto the market it's going to need assistance to gain credibility and establish traction, so start-ups often set aside some of their tokens for just this purpose.
Since word-of-mouth advertising is powerful, bounty programs will often target the crypto community in these and other ways:
Social Media Bounty - they will use the social media accounts of participants to promote the ICO. Likes, reposts, shares, views, and comments on popular platforms like Twitter, Facebook, YouTube, and WhatsApp all help to build momentum.
Content Marketing Bounty - content creators, or influencers, provide their audiences with information and can be hugely popular. Start-ups will encourage creators to produce articles, blog posts, or videos to get the word out. Usually, the better the results they achieve, the bigger their rewards.
Bitcointalk Signature Bounty - These are open to Bitcointalk forum members. They just need to add the official ICO signature to their profiles. The bounty that participants get will typically depend on their rankings, and it's usually only open to those ranked as Jr. Members or above.
Translation Bounty - This bounty consists of translating important documents related to the project in order to ensure global reach. This commonly includes the ICO Whitepaper, the official website, and the Bitcointalk ANN thread.
Bug Reporting Bounty - by encouraging developers and security researchers to find bugs and vulnerabilities in the blockchain infrastructure, they motivate some highly effective community product testing. The more severe and significant the bug, the greater the bounty.