Refer to figure.
VDF (VHF Direction Finding) is a method of measuring the direction from which a VHF signal came. It has been used for many decades now by ground stations, and is particularly useful as the transmitting aircraft only needs a standard VHF radio to ask for a VDF bearing from a suitably equipped ATSU. This can give them:

QDM - Magnetic bearing from the aircraft to the station.
QDR - Magnetic bearing from the station to the aircraft.
QUJ - True bearing from the aircraft to the station.
QTE - True bearing from the station to the aircraft.

The antenna for a VDF ground station is called an Adcock aerial. It is a series of standard dipole antennas in a circle, each receiving the incoming wave in slightly different phase. The system (or operator) can then calculate which direction the signal is coming from. Skilled radio operators can also use the older style equipment to give a classification of how accurate the bearing was, Class A, class B, etc.

This can give the ground radio operator an idea of the location of some aircraft when RADAR is not available, however, that is not a very common usage as far as we can tell.