Plain English Meaning
TACAN is a navigation aid used mainly by the military. It tells a pilot two things from one ground station: which direction the station is from the aircraft, and how far away it is.
Full Definition
A military ultra-high frequency (UHF) navigation system that provides an aircraft with both bearing (direction) and slant-range distance to a selected ground station. TACAN ground stations are often co-located with civilian VOR stations to form a VORTAC, allowing civil aircraft to use the distance (DME) portion while military aircraft use the full bearing-and-distance signal.
Context Anchor
You may see TACAN on aeronautical charts, in military flying information, or near facilities where a military navigation station is part of the navigation picture.