Whether someone is a weirdo, villain, pervert, jerkass, geek, or just way too nice, deviant from the customs of "normal" society — one often finds that those things can only go so far. These characters find that when they're in a situation where they would cross a certain line, they don't do it. Or they get disgusted at those who do cross that line.
The standard is often moral or ethical, but could also be regarding culture, or True Art, or good taste, or good manners, or what constitutes a good meal, or even the boundaries around a fandom — basically, anything that a person or group is willing to treat as Serious Business. The point of this trope is that no matter how high or low anyone thinks the line is, they all believe that there is a line, and that those who cross it are in the wrong. Even people you wouldn't think of as having standards still tend to believe in something.
Compare Conscience Makes You Go Back, Sudden Principled Stand. See What the Hell, Hero? when someone calls out a person's actions for violating their own standards. If someone judges other people's actions but assumes their own must be good by definition, they have Moral Myopia. If someone's standards are applied inconsistently, it's a Double Standard. Conversely, someone's standards may be consistent but at odds with genuine morality, leading to Curious Qualms of Conscience. See also Shades of Conflict for the many variations that may occur when people with different levels of standards collide.
Someone who doesn't observe any standards may cross the Moral Event Horizon and become a Complete Monster.
This is Truth in Television, of course; just about all people do try to follow some sort of standard, and even outright sociopaths can tell (intellectually) the difference between right and wrong. Though standards may differ from time and place and individual, and people may fail to live up to their own standards, for someone to not care about any ethical or moral standards is generally a sign of a severe mental pathology. That said, No Real Life Examples, Please!
