They did good for goodness' sake, rather than being directed to by law or by whim. Such characters would obey the law or break it when they felt it would serve a greater good. They weren't bound strongly to any particular social system or order. Their need to help others and reduce suffering took precedence over all else.
A healer who treated both sides of a fight and somebody who fed the starving in a war zone were both examples of neutral good characters.
- Opposing alignment