Behind the calendar button on the left side of the map screen you can find quests. These are tasks that you can complete to get rewards. The objectives range from popping bubbles of certain color to using certain characters on levels. You can complete these quests by playing any of the levels in the game.
The daily quest will change every day, even if you have not completed the quest. You will only get a reward if the quest is completed in time, so make sure you meet the objective before the day changes. The two other quests change only once you have completed them, so you can take as much time as you wish to complete them.