
I read an author's story and offered editorial comments for free, as a friend. Given that I was subject to abuse and contempt for doing so, this is not a mistake I will make again.
The story had a major flaw that made it unpublishable without fixing: the ending was so unclear that readers would not have any idea what happened or how things actually resolved. My advice that this had to be fixed was ignored, and the story got bad reviews, and the author continues to this day in the belief that he was right, I and the readers were wrong, and nobody understood that he was going for "ambiguity" in the ending.
If your readers don't understand what actually happened, that is not ambigous, that is a lack of clarity. There is a difference.
My favorite example of what ambiguity really is is from a movie I don't much like (though it's a very good film), Clint Eastwood's Unforgiven. (I may go into why I dislike it some other time.)
The climax of the film is all but unforgettable. Eastwood's character has killed virtually everyone standing against him, the main villain, played perfectly by Gene Hackman, lies shot on the floor looking up at his killer. He rasps out: "I don't deserve this. I was building a house."
Eastwood raises his rifle, snarls "Deserve's got nothing to do with it," and shoots him again.
What happens is exceedingly clear. But at this point in the movie, "deserve" actually has everything to do with it. Not only did Hackman's town boss, Little Bill, condone an injustice, he also tortured and killed Eastwood's best friend, leading to the apocalyptic, rain-drenched final shoot out.
So what the hell did William Munny mean by that? That's ambiguity.
Certainly, you can come up with possibilities for what he meant. The most obvious, and thematically attuned, explanation is that Munny had given up being a killer for a decade or so, but Little Bill's civilized savagery had reawakened his killer instinct, after which it did not matter to Munny whether anybody in his way deserved it or not. Another possible meaning arises from earlier dialogue between the Schofield Kid and Munny:
"I guess he had it comin', huh?"
"We all got it comin', kid."
It doesn't matter whether Little Bill deserved it or not, he's going to die, just like everybody else. Could that be it?
And if I watched the movie more recently than I have, I could probably find at least three other interpretations just from the film itself, before venturing further into its larger mythic resonance with Eastwood's career up to that point.
Lack of clarity makes your reader go "huh?" and yanks him out of the story.
Ambiguity makes your reader think and ponder and mull over what happened, and try to figure out exactly why, or what a character meant, or why a character acted in a way that seems contrary to his nature, but also manages to feel "right" even so.
You want to leave your readers pondering questions and interpretations, not scratching their heads and wondering just what in the hell actually happened.