You see the three little icons in the bottom-right corner of your screen, some with numbers on them? Those are the tasks you have to complete for the mini-game. Each one represents some active task for your current session to complete.

Tasks can include killing X number of zombies, completing a quest, and collecting Y armor pieces, killing x number of monsters with y sort of damage.

The images are greyed out at the start and turn colored as you complete each task. If the task requires x number then each time you get 1 the number will decrease by the number. Once all 3 task's have been completed there will by a sound and you will recieve a bonus drop of random items.

Once you have completed the required task's a new set will be generated for you. And you can work towards another bonus reward.