The words grimdark and noblebright arose as technical terms in the gaming world. There's a certain amount of dispute about the exact definitions there, with a tendency to paint them in black and white terms (such as the slur that noblebright is all about rainbows and unicorns and flawless heroes).
In fiction, by contrast, especially adventure fiction (in which I class things like Westerns and Fantasy) they have come to be used to reflect two different and opposed styles of story. Since there is some dispute about the definitions, it behooves me to offer my own.
The notion that the actions of one person can do little to improve this world in decline, that the forces of evil and inertia and temptation will ensure that all of us are doomed. The best we can hope for is a little struggle with morally ambiguous heroes to oppose danger and maybe rescue for a brief time a few others.
The notion that the actions of one person can make a difference, that even if the person is flawed and opposed by strong forces, he can (and wants to) rise to heroic actions that, even if they may cost him his life, improve the lives of others.
Let me explain why I am firmly in the noblebright camp.