The Forum is the centre of your suburb where all of your goods and collected taxes are stored. The amount of a good or resource you can store will depend on the Level of research invested in it.
All of the goods and resources that you can store and the amounts of goods in your forum. To see this you will need to click on your forum and then on this Icon.
Upgrading your Forum will grant additional work permits as well as increase the amount of taxes that can be accumulated there.
In Order to upgrade you forum you will need to click on the forum and this button to upgrade the building.
This will bring up the screen to upgrade your forum. This will show you the resources required to upgrade your forum as well as the benefits you receive from upgrading and the time to upgrade the building.