The docks are used to export goods from your city to the local capital. Once a wharf for a particular good is full, you will either need to upgrade the wharf or export some of the good in order to be able to sell more. To send a shipment, click on the docks, and then click on the icon next to the upgrade icon. The higher the upgrade level of the docks, the more shipments can be sent from the docks at once. To begin sending a shipment, click on the "Send New Shipment" button.
In order to exports goods from the docks, you will need to use Imperial Points. Since Imperial Points only accumulate over time and cannot be obtained with cards, it is important to plan their use wisely!