Laser is an intense beam of light. When it strikes an area, the light is drawn to the darker or pigmented parts. If the darkest part is the hair, then the heat and energy of the light beam will be concentrated on the hair. The temperature will become high enough to kill the hair follicle. But, the laser only works on hair follicles in the active phase of growth.
Hair goes through other phases as well.
Also, different hair follicles' cycles are of different lengths. Some people will have follicles with strong resistance to destruction. First, it will take between three and six or more treatments for the hair removal to be permanent. Each session will catch different follicles in the active phase. Hair that is not entirely removed may grow back finer and lighter.