Settlers are one of the three types of Civilian Units that can be recruited from the Barracks. Their role is helping to establish new Towns and expand an Empire.
Settlers are represented by a variety of vehicles, whether covered wagons or boats, that carry enough supplies to found a new Settlement. As a Civilian Unit, Settlers have no combat utility and cannot damage other Units. However, if a Leader wishes to successfully Conquer an enemy Town, they'll need to take a Settler with the Conquer Mission's army.
Strategy[]
Establishing new territory is dangerous business, so it's important to take plenty of protection along with your Settlers. When establishing a new Town, for example, it's a good idea to send some Units along for defense once it's been established. Don't worry about taking "too many" Units: remember that you will regain 18 Unit Capacity once the Settler does their work, freeing up room to recruit more Units.