These are different forms and types in which a True Neutral character may manifest:
They are morally cowardly, compromisers, or opportunists. A key component of any Untrusting Community; they might believe that good is better than evil, and they'll follow any laws that aren't too much of an inconvenience or angrily mutter about unjust laws in the The Evil Empire, but they just don't have the guts to do anything about it. When the heroes ask about the criminal activities going on, they're not helping, because they know what happens to snitches. They are All of the Other Reindeer. They're the useless masses that villains refer to whenever taunt the heroes by declaring that the masses will abandon them in a heartbeat (something that they typically love doing, by the way,) and it's no accident that True Neutral is the default alignment of humans in almost any fantasy.
Similarly, animals in Dungeons & Dragons are not credited with the ability to make moral or ethical distinctions. Since their existence is amoral and they have no way of changing this, all animals are True Neutral. Often called "Neutral Hungry", they'll leave you alone if you leave them alone (and aren't tasty). This is the reason why the Non-Malicious Monster sub type of Anti-Villain is depicted as True Neutral, as it's merely an unthinking beast concerned with surviving rather than a malevolent destroyer.
They are committed to the philosophical or cosmic principle of Neutrality or Balance itself, and they actively seek to preserve the Balance Between Good and Evil. They only intervene in conflict to prevent the underdog from being wiped out - should their allies gain the upper hand, the True Neutral will probably switch sides from the point of view of his friends; in his eyes, he is keeping true to his own convictions. This can manifest as Chronic Backstabbing Disorder. Stupid Neutral is an extreme version of this.
These types could also be seen as Fence Riding Bastards. People who just can't make a tough (possibly morally gray) decision. So they try to avoid getting put into that uncomfortable position. Could also be both friend and/or Ally to both protagonist and antagonist. Some Actual Pacifists fall into this.
Characters with a very low intelligence will be True Neutral by default. They are simply too stupid to figure out the consequences of their actions, and are utterly unable to put any planning, motives or logic behind them. Any good or evil they might inflict is therefore completely unintentional.
They just live life by doing whatever everyone else is doing. They'll follow the trends, and follow the law whenever the law is generally followed, but believe that if everyone else is doing it, it can't be wrong, right? They're never the first to do anything, but might get caught up in an I Am Spartacus moment once the True Companions have started the call, or grab a rock once the Untrusting Community finds out that there's no Hero Insurance to cover the damage. Can also be called "True Average."
Machines and robots that are limited by their programming are True Neutral. Sapient machines capable of independent thought may have other alignments, but a machine whose ethical and moral framework is decided by its programming is inherently incapable of making ethical or moral decisions on its own and thus is inherently neutral. The same applies to magical constructs or enchanted items that have intellect but no real will of their own, or only the will to fulfill their function, acting only as tools.
They just want to be left alone, to enjoy life for themselves and possibly family, and let other people do whatever. If someone defies that, though, usually villains, they'll fight back. They may have once been committed to a cause, but have gone through so much strife and been betrayed or failed so many times that they simply don't care anymore, and will only focus on their own goals and help or hurt other people if they feel like it. This can be a more tempered variant of Vigilante Man.
They are dedicated to a particular side, but they have tendencies of all alignments at the same time. They may be good in one area and evil in another. They aren't neutral in the sense of not taking sides; it's simply that the sum of their actions is between good and evil and order and chaos. Anti Heroes and similar characters who live by Gray and Gray Morality are often this kind of neutral.