Taxes

After constructing the Treasury building, you'll be able to collect a daily tax that is calculated based on your city's population, and the level of your Happiness. The treasury at City Hall has a limited capacity, so you should visit to collect taxes from it every day by tapping the Coin icon above the building.

They are how your citizens pay their taxes; you receive them as rewards for keeping all things  good and for building & upgrading your city. Take care of your citizens!

Start earning Coins by placing residential homes. As your city population expands you’ll have more opportunities to upgrade those homes, collect taxes, complete cargo shipments, make quests. Each of these actions will keep the Coins rolling in. If things get tight you can also convert your Nano Bucks  to Coins, purchased Coins in the store.