What is light pollution?
Light Pollution is excessive, misdirected, and obstrusive (mostly outdoor) light. It is usually evident in cities in night where excessive artificial lights is used in night.
Too much artificial light washes out the starlight in night sky, disrupts ecosystems, create problems for nocturnal animals such as owls, interfers with astronomical research, has adverse health effects, and wastes energy.
If you live in cities you can never experience true dark sky. Even in late night if you observe from your balcony or roof you will observe a faint glow in the sky. This is the light pollution due to artificial light.