Missions are a feature that will allow you to earn coins by fulfilling different objectives within the games.
The requirements of each Mission can be found on the Main Screen, where it says "Missions".
Bear in mind that you have 24 to fulfill the 3 available missions. After 24 hours go by, the tasks for the next mission will change.
Once the tasks are completed, you'll have to go to "Missions" to claim your rewards.