You know how sunny days heat up dark-colored cars faster than light-colored ones? The same science behind that difference is what powers laser hair removal.
A specialized device emits light energy into the skin, where it rapidly heats up the darker-pigmented cells in your hair follicles. This sudden heat damages the follicles so they can no longer properly grow new hair.
While this removes the current hairs, this will not prevent new ones from growing back. Instead, it will take much longer for new growth to begin, and any new hairs that form will be much lighter and thinner than your current hair. Many call this treatment laser hair reduction for this reason.