Other people have answered a couple of different interpretations of your question, so let me cover another in case this is what you wanted to know:
If you have a resource bonus square in your territory, like horses or iron or whatever, in order for a city to have access to this resource to produce whatever requires it, you'll need to make a road connecting the resource square with whatever city you want to have it. Note that the resource 'travels' down your road network, so if you connect your resource to CityA, and CityA is connected to CityB and CityC, then all three cities can now build things requiring that resource.