Missions are time-based game modes that send you on a quest to earn special rewards. Completing missions is the best way to earn upgrade materials for items and heroes. 

There are a variety of missions that offer different types of rewards, sometimes your mission is to take on the toughest bosses and other times to defeat tougher but smaller enemies. 

Each mission can also be played at different difficulty levels. The difficulty you choose will result in more or better rewards. To unlock the next difficulty, you must complete the mission of that difficulty once. The next time you play the mission, you will be able to choose the next difficulty level.

If you don't manage to complete the mission in the given amount of attempts,, you can try again by recharging your mission with gems. Recharging will not allow you to receive the rewards again.