Plain English Meaning
A radio beacon on the ground that lets you fly directly toward it or away from it along a chosen line. Your cockpit instrument tells you if you are on that line or drifting off to one side.
Full Definition
A ground-based radio navigation aid that transmits VHF signals allowing a properly equipped aircraft to determine its bearing from the station along any of 360 selectable radials. The pilot tunes the station's frequency, selects a desired course on the cockpit indicator, and the instrument shows whether the aircraft is left of, right of, or on that course relative to the VOR ground station.