These lines are most likely low-light image noise from the webcam boosting a very weak signal. In a dark scene, the camera’s automatic gain/ISO tries to brighten the image toward a normal mid-tone, but with almost no real light hitting the sensor it mostly amplifies noise instead. That noise can show up as moving grain or faint horizontal banding/lines.

The practical fix is to give the camera more light so it doesn’t need as much amplification. Brighter illumination will usually reduce the effect a lot. On many webcams there isn’t much manual control, so in very dim light this behavior is normal rather than a repairable fault.