Successfully completing levels in a row grants a win streak bonus.
Each time you clear a level, you'll receive a free item that is automatically applied at the start of the next level. 
When you successfully complete a level, the reward you receive at the start of the next level increases with each consecutive success, up to a maximum of three in a row.
If you clear 3 levels consecutively, you'll start the next level with the maximum number of reward items.
If you fail a level, your COCO's Tray rewards will be reset.

COCO's Tray unlocks at Level 30.