Shards are used to unlock new heroes and increase a heroes' rarity.Unlocking new heroes can be done by collecting a certain amount of Shards, the most common amount needed to unlock a hero is ten Shards, but there are special circumstances where many more shards are needed.
If a hero requires more Shards than normal to unlock then that hero will also be unlocked at a higher star level than normal.
Shards are rewarded for completing certain missions in the Campaign mode.
You can also buy them in the Shop.
To find out which missions reward Shards, tap on Search.