What is light pollution and just how detrimental is it?

What is light pollution?
According to National Geographic, light pollution is the excessive or poor use of artificial light outdoors. Types of light pollution include but are not limited to, glare which is the excessive brightness, causing visual discomfort; skyglow which brightens the night sky over populated areas; light trespass when artificial light is directed into not intended or irrelevant spaces; and clutter which is the combination of different light sources that is too bright, excessive or confusing. Light pollution is a huge but often unrecognised global issue.