Bounties are located at the top right, 4th over from the right. These provide various buffs such as loot drop, monster essence, experience, gold drop, etc. These last for 2 hours, and you can either roll for the monster you are looking for with Bounty Cards or choose the select option.
You start off with 1 slot unlocked. Each additional slot is 150 Bounty Cards and is a 1 time cost.
You can additional perform additional options in the Bounties menu.
Bounty Cards drop through the Arch Pass (Free and Premium), through Loot Slimes, Prison, and quests. You can also purchase these through the market.