Light pollution is the artificial light, then emitted by humans, which is generated without a criterion of efficient lighting.
The word "pollution" highlights the fact that it is an excess, something in the environment that is detrimental to it. "Light", which is sometimes used in a positive sense, here understood as the scattering of light and causes unnecessary pollution.
The evolution of our cities requires us, for commercial reasons and security, to illuminate streets, parks, businesses, shop windows and whatnot, but the misapplication of an efficiency criterion in this lighting results in the spread of light to excessive that does not serve to illuminate (the sky, the trees).
To understand what it is, just think of the classic "balls," lighting the drive way or public parks.
These generate a large portion of their light (over half) to the other, toward the sky, which of course is not to be illuminated. In addition therefore to unnecessary waste of energy, we are leading, slowly, the loss of that which is the spectacular starry sky.
Just use light sources that light only downward, there are already enough to use it!