Provinces are specialized to harvest one particular resource. Therefore there are different resource provinces for every resource. You can either raid or conquer provinces, although you can only conquer a province once you have upgraded the watchtower to an appropriate level.

You can harvest the resources of your provinces, which will be added to your castle's recources. Also, you can send units to protect your provinces, as there might be other Lords that want to conquer them. If you want another province or just don't need yours anymore, you can abandon it.

All these actions can be initiated by clicking on the province and choosing the possible options.