Every turn you own a city, it's production is added to it's storage.  By looking at the map, you may notice cities tend to specialize in one or two commodities.  What this means is that in order to maximize production, you'll have to cart commodities from one city to another.  Many armies have specialized units which perform this function. With other armies, you have to divert fighting units to this task.  

Generally this is done by transferring commodities from a city to a unit, moving the unit to another city and transferring the commodities to that city.  You can do this all through the commodity transfer dialog, but there are also some quick commands for loading and unloading. All of these commands can be found in the Commands menu.

Quick Load: Transfers as much of the specified commodity from the city to the selected units as possible. It stops when the selected units are full or the city runs out of the commodity.

Unload: Unloads all commodities from the selected units to the city.

Unload All: The ultimate unload. Unloads all commodities from all units stacked in the city regardless of whether they are selected of not.