There are three types of resources: Gold, Food and Supplies. Each building brings you one type of these resources when you play match-3 levels or collect taxes. For example, the Field gives you food resources and the Hut brings you gold resources. On match-3 levels in the buildings, it's better to match tiles that bring resources. Some tiles are more valuable than others. When matched, they bring more resources. You need to upgrade your buildings to unlock more valuable tiles.
You need resources to do the following: