Coins are an in-game currency used to purchase upgrades for your heroes, weapons, armor and so on!

Coins can be found by going on a run, from loot boxes, patrolling, the Growth Fund, events and the list goes on! So by playing the game, you're sure to encounter some coins. 

Coins can also be purchased by using gems from the shop, so if you're ever down on coins you may consider buying some with the gems you've acquired.