There are various in-game resources that can be used to purchase items, upgrade, and battle with.
In-game Currency
Credit - This is one of the main resources in the game that is used to upgrade skills, enhance gears, etc.
How To Obtain Credit:
Credit can be obtained through normal gameplay (clearing stages), exchanging with Hero Coins, etc.
Hero Coins - This resource is most commonly used for recruiting Heroes.
How To Obtain Hero Coins:
Hero Coins can be obtained by clearing certain missions, events, and in-app purchases.
Other Resources
Stamina - Stamina is the main resource used to go into battle.
How To Obtain Stamina:
Stamina can be obtained by waiting for it to recover, exchanging Hero Coins for Stamina, using Consumables to restore Stamina, gifts from Friends, and through Dorm Furniture.
Hero Shards - Shards can be used to recruit a Hero you don't own (with enough pieces) and is also important for Resonance.
How To Obtain Hero Shards:
Hero Shards are obtained through Hero Recruit, Hero Trials, etc.
Food - Used for training your heroes, to improve their stats.
How To Obtain Food:
Food can be obtained through completing stages, and Supply Battles.
Drones - Can be used in Peacekeeping, allowing you to Rush through stages.
How To Obtain Drones:
Drones can be obtained through the mall.