The process of laser hair reduction involves targetting laser beams to the growing hair follicles under the skin. The melanin pigment in the follicle absorbs the light and releases heat to damage the growing hair cells and reduce hair growth. This works mainly on hairs which are in the growing phase. That is why some hair grows back again following each session (the ones in resting phase).
Sessions need to be repeated after 1-2 months to target the next cycle of hair growth, and sessions are continued till most of the follciles have reduced or no growth. This can take 6 or more sessions depending on your body part and hormone status e.g. facial hair in women with polycystic ovary may need more sessions along with hormonal treatment while underarm hair in other females can respond in less time.

Once the growth is reduced more than 90%, the few hairs which occasionally grow become very fine and sparse. Yearly maintenance sessions may be needed to keep growth at zero, although clearance of full areas without need for maintenance is also common.