Laser hair removal uses concentrated beams or "pulses" of light to destroy hair follicles. The light is absorbed by melanin, the pigment that gives skin and hair its color, so it works best on people with light skin and dark hair.
That's why lasers won't work at all on white hair and don’t work well on blond hair. (It's also wise to avoid tanning for 6 weeks beforehand.)
For dark skin, laser hair removal is best done using a device with longer light wavelengths, to reduce the risk of burns and discoloration to skin around the hair follicles.