Roads is a Tier Two technology after Riding that allows you to place roads to connect cities, where each "piece of road" costs 2 stars. Cities that are connected to your capital city gain 1 population. Your capital city gains 1 population for every city that is connected to it in return. Note that cities can also be connected via ports. This means you can connect cities without researching the Roads technology and still get the population bonus.
Roads unlocks the Network task.
Connected roads in friendly or neutral territory also reduce the movement cost to friendly units travelling along them to 0.5. Roads in enemy territory won't give you a bonus.Roads expands into Trade.