The Treasury is where you store your coins. Level it up to expand its capacity. Coins can be used to train Dragons, hatch Dragons and upgrade buildings.
This building will generate more Coins over time. Clicking on it will show the production rate. Click on Upgrade to see what the current rate of Coins per hour is, how much it can hold, and how much an upgrade would increase.
The image for the Treasury will be different depending if it's full, half full or empty.