Just like a light switch controls the flow of electricity, there is hardware at the end of fiber optic cable to control the waves of light on which information travels. Fiber optic cable is the same physically whether or not it is lit. The only difference between dark fiber and lit fiber is that lit fiber has equipment to control light-waves.
Dark Fiber = More options for you. Building new infrastructure is expensive, and previous fiber optic networks were typically constructed (dark) and operated (lit) by one company. By assuming the cost of construction and not lighting the fiber with equipment, Metronet can be vendor neutral; this means more choices of both vendors and services.