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Light pollution is light from cities and other human-made light sources that raises the level of light at night causing problems. The issue for astronomy is that light scattered in the atmosphere cuts down on the clarity of optical observations (not as bad as daylight, but enough to limit astronomical observation within or near a city). There are also non-astronomy issues with such light, e.g., disturbing the nocturnal habits of wildlife.
Observatories and research telescopes are generally located far from cities because of light pollution, and some older installations have lost considerable value for astronomical observation. Also, most people today do not see the night sky with as much clarity as in times past or from locations far distant from artificial light sources. To minimize the astronomy problems of light pollution, some urban/suburban neighborhoods of research observatories are illuminated by sodium street lamps, which produce virtually monochromatic light (a fine structure doublet of about 583 nm wavelength) which can be filtered or ignored. A current issue regarding light pollution is satellite constellations, which produce observation-challenges even in places that have minimal light pollution from cities, etc.
Radio astronomy has the analogous issue with human sources of radio for communications, radar, etc., generally referred to as radio frequency interference (RFI).