The order board allows you to trade items you produce for coins and experience points.

To start trading, tap the board and select an order from the map that will appear. To the right of the map you will see a list of items required to fulfill the order. Tap the "Send" button under the list to complete the trade.

You can also skip an order by tapping this button:

Completing multiple orders will reward you with a chest containing energy, coins and growth elixirs. Your progress towards the next chest is represented by the bar at the bottom of the map. When the progress bar reaches the end, you will receive a chest that also contains some diamonds.