This is an Event in Gems of War. Events are of limited duration. When there is an active Bounty Event, you can find it under the Games tab.
Unlike all other Events, you can just keep playing a fight you like over and over. You aren't required to advance to higher levels in this Event. This means if you get in over your head, you can go back to an easier fight and grind to your heart's content, assuming the Sigils hold out.
You will get a lot more Rewards if you sort troops by "bountyhunter."
It's a trait. If you have purchased that trait, each bountyhunter troop increases your points towards Rewards when you win a fight.
The Bounty Event is a good Event for getting a bunch of Rewards. That's basically all there is to this Event, as far as I know. Other Events can be tied to your Guild's performance, can feature a particular Kingdom's Troops or have specific limitations.