Plain English Meaning
A control tower at an airport that handles traffic by looking out the window rather than by working radar. It runs the runway and the airspace right around the airport, but the radar work for instrument flights is done by another facility.
Full Definition
Air traffic control towers that manage traffic at airports under visual flight rules but do not provide radar approach or departure control services. VFR towers sequence and separate arriving and departing aircraft visually within their airspace, and hand off IFR aircraft to or from a separate radar facility (such as a TRACON or ARTCC) that handles the instrument portion of the .