Final answer:
The frequency of 121.5MHz is the International emergency frequency, intended specifically for use in aeronautical emergencies. Its use aims to prevent the interference with crucial equipment, similarly to why we switch off cell phones in certain environments. The selection of frequencies for particular uses needs harmonization to reduce interference.
Step-by-step explanation:
The frequency of 121.5MHz is the International emergency frequency. It's utilized for emergency communications globally, mainly in aviation. It is used for emergency communications and distress signals. When someone is in trouble and needs help, they can transmit a distress signal on this frequency to get assistance. This radio frequency is regarded by the International Civil Aviation Organization as a frequency to be used in aeronautical emergencies.
The international emergency frequency is 121.500MHz in the VHF band. It shall be used for emergency purposes only.
The emergency frequency is called GUARD in IVAO ATC client. GUARD is an old denomination that should not be used any more (Rules of the Air).
The use of such frequency helps prevent interference with important equipment--similar to why we're sometimes asked to turn off our phones operating at frequencies close to 1.9 GHz in airplanes and hospitals. Like the resonance of currents on antennas that match 1/4 their wavelength or the similar fundamental resonant mode of an air column closed at one end, the selected frequency for specific uses has to be set not to interfere with others.
Learn more about International emergency frequency