To expand on xenoids answer.
Most phone cameras use what is known as a "rolling shutter", the exposure starts and ends at slightly different times for different parts of the image. This makes the sensor cheaper because the end of the exposure can be defined by the readout process rather than needing extra electronics to capture the image at the end of the exposure.
This causes time-variations in the lighting level to be translated to spacial variations in the resulting image.
So if your light source varies in intensity at a speed a few times faster than the sensor readout time, you will get bars like this. How dark the bars are will depend on the exposure time the camera is using. Pointing your camera directly at the problem light will likely result in a shorter exposure time and hence stronger bars.
Many (but not all) flourescent and LED lights flicker at twice mains frequency, which tends to be in the same ballpark as sensor readout times.