Control is an established, measured, and documented point for surveys and mapping. Surveyors determine other positions relative to the control points.
Survey ["station"]s are established for different purposes; some stations are for horizontal control and other stations are for vertical control. Horizontal control is needed for construction projects to make sure the construction happens at the proper place on Earth. Vertical control is needed for such things as railroads and bridges to make sure that they are not changing in ["elevation"] over time.