The top banner displays your current balance of coins and your coin production rate in coins per second (also displayed as cps). The display shortens the digit length by displaying 1,000,000 as 1.0 Mi, and 1,000,000,000 as 1.0 Bi, etc.
Coins can be used to purchase platypuses, with the prices increasing the more you buy. There are also game upgrades in the shop that can be purchased with coins, for example the tractor.
Purchasing the tractor from the shop allows coins to be collected for up to 2 hours while the app is closed. Purchasing the tractor upgrade (again with coins) extends this time limit to 4 hours.
There are also in app purchases (i.e. they cost real-life money) which allow you to instantly double your coins, or double your coin production forever. Coin production also permanently increases by 1% each time the universe is reset.
In addition, a coin bonus can be collected each time the fusion bar in the top banner is filled, with the bonuses increasing each time. The bar gradually fills each time two platypuses are fused. Once full, birds fly across the screen and their poop disappears and counts as coins in the same way platypus poop does.