MASERs (Microwave Amplification by Stimulated Emission of Radiation) are devices that produces coherent and highly amplified electromagnetic waves in the microwave frequency range. A MASER consists of a resonant cavity and a medium that emits microwave radiation. The medium is composed of atoms or molecules with specific energy level transitions. These transitions can be stimulated by an external energy source, such as a high-frequency electromagnetic field, which causes the atoms or molecules to emit coherent microwave radiation.