When the laser light is absorbed by the hair follicles, the energy is converted into heat. This heat damages the hair follicles, preventing them from producing new hair. Over time, the hair will slowly stop growing, resulting in smoother, hair-free skin.
The process is typically done in multiple treatments, as hair growth occurs in cycles and not all hairs will be in the same stage of growth at the same time. But, with each treatment, the amount of hair that grows back will be less and finer, until the hair is effectively gone.