Roads is a Tier Two technology after Riding that allows you to place roads to connect cities. Connected cities get 1 population as a bonus. Note that Cities can also be connected via ports. Each road costs 2 stars.
Roads unlocks the Network task.
You can place roads on anything except for mountain, water, and ocean tiles, even on tiles outside of your territory but you cannot place roads on tiles in an enemy tribe's territory.
Connected roads also provide a +1 movement bonus to units on them.
Roads expands into Trade