What does "dark sky compliant"mean when installing an outdoor light?

Designed to limit light pollution.

Simplified, it means that the light does not radiate upward, thus limiting artificial lighting of the night sky.

New Mexico, as a state, has adopted dark-sky compliant lighting state-wide. The entire city of Santa Fe, New Mexico, for example is lit entirely by dark sky compliant lighting. So you can see the stars at night.

It means that the fixture is designed to put light only where and/or when it's actually needed, rather than lighting up, for example, the house across the street as well as your house, or 50 feet up into the trees.

It's an outdoor lighting fixture that meets the International Dark-Sky Association’s (IDA) standard for reducing light pollution (or in other words, minimizing glare) into the night sky.