I'm not a physics type, so optics
specialists please read with a hawkish eye for corrections!
A substance lases, in the barest layman's terms (i.e. the maximum of my knowledge) when the molecules or atoms of an excited volume of the lasing material drop from an energy state higher than their' 'rest state' back down and, in the process, emit energy quanta in the form of photons of a specific frequency. These photons form a beam of coherent light (laser light) whose defining characteristic is that all light in the beam is of the same frequency, and all of it is moving in the same direction.
This is an ongoing process, such that as energy is fed into the system by means of flashlamps or other methods the lasing material will continuously climb and descend the energy state ladder, emitting a steady beam of laser light until the energy source is removed. Since it takes a certain fixed level of input energy to reach the excited state, the laser will not 'fade' as power fails (batteries, for example) but will instead either begin to flicker on/off as the input energy level oscillates across the lasing threshold, or perhaps drop to a dimmer level if, due to the construction of the laser, a smaller amount of material remains exposed to the same energy level instead of the entire system experiencing a drop in input energy.