... refers to an especially quiet band of the electromagnetic spectrum between 1,420 and 1,666 megahertz...Within this frequency range, interference (attenuation) due to interstellar water molecules is at a minimum, making it a good place (supposedly) to look for signals from other civilizations.
The irony is that "water holes" are places where different species of animals meet and gather in the (terrestrial) wild.
Interesting!