Negative resistance device - typically a "tunnel diode".
They do not have "negative resistance" such that when a voltage is applied the current flows the opposite way!
But, there is a "kink" in their current-voltage curve, so that at some point as the voltage is increased, the current will decrease slightly instead of increasing.
In the circuit you have shown, adjusting the potentiometer, will give a voltage which will make the circuit oscillate so that you can hear a tone in the headphones.
JimB