A hidden item can be common, uncommon, rare, very rare, or legendary. Also, the item will have a certain level of quality. Quality ranges from 1-star quality to 5-star quality. For example, a Rose Quartz is a common gem in Treasure Temple; you may find a 1-star Rose Quartz or a 5-star Rose Quartz, 5-star is the best. Try to find a 5-star quality of each item to complete your collection!
Every time you uncover a higher quality version of an item during a round of Bingo, your Collection screen will update to show the new star rating you have earned for that particular item.
The Star rating of each hidden item you uncover during a round fills up your Star Progress meter. You can increase your Daily Bonus cherries by +1 by filling up your Star Progress meter.
For more on the Daily Bonus please see "How Do I get More Cherries"?