Bounties are a way for your guild to help each other earn guild coins.

There are different kinds of bounty challenges you can choose from. Please, note that bounty list refreshes every 25-30 minutes. 
Bounties have a 3 star rating systems that reflects their difficulty. The more starts for the bounty the more difficult it is to complete. 

Each start to a bounty will give you around 10 guild coins upon completion (so 1 star bounty is worth 10 guild coins while the 3 stars bounty will give you 30). 

Once a guild has reached 100 stars everyone will be rewarded with 300 guild coins. This can be repeated as many times as your guild can complete them.