A Neutral character might believe in the coexistence of Good and Evil: while a Good adventurer might scour it from the land, a Neutral character might not fight Evil where other options are present. A Neutral character might believe in the coexistence of Law and Chaos: that a society, for example, cannot function without both.

D&D by default seems to assume that Good characters are truly Good, and that if you commit murder on a surrendering goblin, for example, you must be Evil. To better frame the alignment system, you have to look at the alignments as different sides, albeit sides where those in the camp of Good pursue selfless, pure objectives and those in the camp of Evil pursue selfish, impure objectives.

Good people can do bad things, Evil people can have friends, and Neutral characters might want differing philosophies to coexist where each philosophy may not.

A Neutral character might also be averse to conflict, meaning that they want to put an end to conflict between Good or Evil, even if there is not a decisive victor, Good or Evil. So, another interpretation of a True Neutral character might be just that: wanting neither side to win, not necessarily for the sake of the status quo (although that might be the case), but for other reasons: to protect the world from conflict, because on a cosmic scale such conflict cannot be prevented in the long-term and each side's effort is better used elsewhere... there are many options.