It allows you to build roads between cities to form City Connections. Each Road costs 2 stars per tile. 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 also unlocks the Network task.
Roads expands into Trade.
Roads in friendly or neutral territory also reduce the movement cost to friendly units travelling along them to 0.5, allowing units on Roads moving onto other Roads double movement. Roads in enemy territory won't give you a bonus.
It is impossible to remove a Road, even with Destroy.