The "Settlers" unit is one of C-evo's nine predesigned units, available right from the start. It can improve terrain and it can settle down to create a new city. (It needs little introduction to players of Civ1 or Civ2.)
With permitted movement points ("MP") of 1.5, a single turn can be one step in any direction on any terrain, two or three steps along roads or rivers (but it needs 1.0 or 1.5 MP to go from road to river or vice versa), and several steps along a railroad - where a unit of that type uses only 0.12 or 0.18 MP per tile, depending on direction.
The time a settler takes to perform land improvement tasks depends on the task and the terrain. The shortest take two turns. The in-game manual refers readers to a "terrain enhancement box". See this wiki's version.
Once you discover Explosives, you can instead create Engineers units, which have twice as many MP for the same construction cost. At that point, there is no value in building settlers; in v 1.0 you have to "obsolete" them yourself, but in v 1.2 that is done automatically.