To build units or buildings you need resources. You receive those resources automatically from your resources production (Timber camp, clay pit, iron mine). You also receive those resources when you are not logged into your account.

The amount of resources that you get depends on the size of your resource buildings. It is important to build up your warehouse to make sure that you don't lose the resources that don't fit. On the top right you see how many resources your warehouse can hold, and how many you currently have.

The number on the right shows how many resources can be stored each. In this example 1000 resources can be stored, this means, 1000 wood, 1000 clay and 1000 iron. The numbers to the left of that show how many resources are stored right now.