A maser, even under the best of circumstances, is a device of questionable utility. Maser stands for Microwave Amplification through Stimulated Emission of Radiation. It is similar to the ubiquitous Laser, except that it uses microwaves instead of visible light. These were first invented back in '53, a decade before the first lasers.
Masers are not terribly useful. They require powerful magnetic fields and a strong cooling system to function. These make them bulky and expensive, which limits their usefulness. They do have some applications: communication with deep space probes, among other things. However, they could only be used for a very narrow range of things. For many years they sulked in the dark corners of physics laboratories, unwanted.
They may finally get their chance to shine. Scientists have developed a crystalline material, that when bombarded with an ordinary laser, emits coherent microwaves. This new design can probably be built without the need for a bulky cooling system or costly magnets, which may make it much smaller and cheaper.
If Masers ever became as ubiquitous as lasers, they could have many applications. Some of these are:
Extremely long distance, high bandwidth data links
They might be usable for conducting laser surgery inside the body without having to make an incision. This is because they pass through the skin. You could focus several on a point inside the body and cut out a tumor from within.
They may have some applications as weapons.
It remains to be seen whether the Masers will achieve any of these. We'll see. Now is their time to shine.