A reward is the loot you recieve from defeating an enemy. They are given in reward chests. There are three types of chests: bronze, silver and gold.
Bronze chests are rewarded when you are defeated. They contain the least valueable loot. Bronze chests contain 1-7 sapphires or an emerald. They don't contain tokens.
Silver chests are rewarded when defeating a rival on the same difficulty in practice mode for the second time. They are also obtained from defeating rivals in normal mode. Silver chests contain 5 or 6 tokens and some crafting materials.
Gold chests are the best chests you can get. They are obtained when you defeat a rival in practice mode for the first time in that difficulty. They can also be obtained in normal mode from defeating rivals ,however there seems to be a limit to the amount of gold chests you can recieve per day.