This video talks about negative resistance. Static resistance determines the power dissipation in a component. Passive devices, which consume electric power, have positive static resistance; while active devices, which produce electric power, do not. But usually when in electronics you talk about negative resistance you mean negative differential resistance which can be found, for example, in the cross coupled pair. Negative differential resistance means that if you increase the voltage the current through the device decreases, in contrast to ohm’s law.
Source Coupled VCO : https://youtu.be/GhrJun8bg08
Miller effect made easy: https://youtu.be/TD3jhSzOdqI
