Plain English Meaning
A simple ground radio station that sends out a steady signal in every direction. An aircraft instrument picks up that signal and shows the pilot which direction the station is from the aircraft, so the pilot can fly toward it or away from it.
Full Definition
A ground-based radio transmitter that broadcasts a signal equally in all directions on low or medium frequencies, used as a navigation aid by aircraft equipped with an automatic direction finder (ADF). The aircraft's ADF receiver senses the direction from which the signal is arriving and points a needle toward the station, allowing the pilot to track to or from the beacon.