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 still a reduction), but there can be unrelated issues with such light such as 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, much of the population does not see the night sky with as much clarity as in times past or from locations far distant from artificial light sources.
Radio astronomy has a similar issue with human sources of radio for communications, radar, etc.