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.

With 2 suitable bearings from different ground stations, there is enough information to calculate a location of an aircraft, whether that is the pilot drawing on a chart and finding the location, or the controllers on the ground using their system. More bearings provide a more accurate location, and the best bearings are where the lines cross at close to 90º. This means that the two VDF ground stations are required, that are not too close together.

These days, a computer system with access to multiple VDF aerials can perform a process called auto-triangulation to find the exact location that a particular transmission was broadcast from. This is most notably used by Distress and Diversion (121.5 MHz).