Coins are an in-app currency that is Long Game's way of rewarding you for saving. You can use Coins to enter into instant games and the weekly drawing.
You earn Coins based on how much you are holding with Long Game. Coins are earned on an ongoing basis based on how much money you are saving in your Long Game account.
Additionally, you can earn Coins by accomplishing savings Missions. You can see full earning rules here: https://www.longgame.co/terms-conditions/coin-earning-rules/