Light trespass is the crossing of a property line by artificial light.
It is wrong because it violates the rights of those across the property line. It does this by vastly reducing the night vision of those off the property. It can create pain and discomfort for those looking toward it. It creates a hazard on the road by obscuring pedestrians and other objects from a driver's view. Ironically, it does this while creating a false sense of security for pedestrians who can mistake all the light around them for an assumption that a driver is aware of them.