"Neutral Good" by d&d rules would mean a hero who, while being more or less ambivalent towards rules, wouldn't actively oppose them either if they could avoid it. Contrasting the reactions to the situation of, say, their king passing a law the character knew to be unjust, a Lawful Good character would have a crisis of conscience over whether to obey or not, a neutral character would disobey as far as it didn't cause them trouble, and a chaotic character might well go out of their way to encourage rebellion against the law.
"Neutral Evil" is, similarly, a villain who follows the rules exactly as far as it benefits them to do so. Lawful villains might abuse loopholes for all they were worth but would rarely outright violate the law, neutrals would follow the law publicly because it's less trouble to not be in jail constantly, and chaotic villains park their cars in fire zones just because they can (slight exaggeration).
If you're a neutral neutral character, it generally means your creator is too lazy/wishy-washy to stick with one alignment, or else playing a druid.