Artifacts are unwanted effects on film or in videos, like wagon wheels that appear to slowly spin backwards, even though they were spinning rapidly forward.  Another term for this is “beating.”  It is caused by interference between the rate something is actually changing in real life and the rate at which it is being captured.  If an LED dimmer uses pulse-width-modulation (PWM) for dimming, it is actually flickering the LED faster than eyes can see.  Unless it is very fast, it is not faster than a camera can see.  The result will be visible artifacts.  The most common artifacts caused by PWM dimming on camera are “banding” and “strobing.”