In Brawl Stars, there are 2 types of currency for players using in the game to buy things like Boxes and upgrading their brawlers.
Coins are used to upgrade your Brawlers. After you collect Power Points to unlock the next upgrade of Brawlers you also required Coins for using to upgrade it.
Coins also can use to buy Power Points of any brawlers in the shop Daily Deals, the shop will be refresh everyday.
There are 2 ways for you to get Coins in the game. First, you will get Coins by opening any Boxes in the game; Brawl Box, Big Box, or Mega Box.
Second, you can use Gems to buy Coins in the shop Coin Packs. 20 gems can buy for 150 coins, 50 gems can buy for 400 coins, 140 gems can buy for 1,200 coins.
Gems are used to buy Boxes, Skins and other things from the shop.
You can get Gems from Boxes or purchase from the Shop with real money.