Plain English Meaning
A VORTAC is one ground station that does the job of two navigation aids at once. It tells civilian pilots which direction they are from the station and how far away they are, and it serves military aircraft at the same time.
Full Definition
A VORTAC is a single ground-based navigation facility that combines a civilian VOR (VHF Omnidirectional Range), which provides magnetic bearing information, with the military TACAN (Tactical Air Navigation) system, which provides distance information (DME) and its own bearing data. Civilian aircraft use the VOR portion for bearing and the TACAN's DME portion for slant-range distance, giving them both and distance from the same station. Military aircraft equipped with TACAN receivers use the full TACAN signal for both bearing and distance.