A Marconi Direction Finder is a radio direction finder from the very early days of radio. I’m guessing on the details because I’m too lazy to look 'em up. But I would guess there’s a directional antenna someplace (probably a large loop) that should attach to this device, probably mechanically but maybe it drove an electric motor (need to see inside). You turn the knob and the directional antenna rotates proportionally. The signal strength has to be monitored somehow (would probably have been done “organically” by an operator with an earphone), and the bearing to the radio transmitter would be the direction the antenna is pointing when the signal is strongest. It probably would have been referenced to the front of a ship, so 0º would be dead-ahead. The radio operator could determine the direction to steer to intercept a ship in distress. “Steer 37 degrees to port!”.
This type of system was the primary method of locating ships at sea for a good part of the 20th century. The technology obviously would have improved significantly, but the basic idea of pointing a rotating antenna at the source of a transmission to determine the bearing to that source didn’t change until everyone started using GPS.