Hero Coins are currencies in AFK Arena. You can get Hero Coins in the Rickety Cart from retiring Common(green) tier heroes.
You can retire heroes manually or change the switch in the Rickety Cart to retire them automatically. For retiring single Common hero you will get 160 Hero Coins.
You can spend Hero Coins in the Barracks Store, where you can buy 60 Elite Soulstones for 18 000 Hero Coins, 60 Rare Soulstones for 2400 coins or Angelo, Ankhira, Ogi or Niru for 4800 Hero Coins each.