A: When I think of high-concept fiction, I think of blockbuster-movie-kind-of-fiction. If you’ve got a “coming of age” story, where everyone is a regular human being that goes to a normal school, the girl breaks up with a bad boyfriend, but then realizes she was worthy without him, then that’s probably not high-concept.
If on the other hand, your story is about a team of undercover teen spies working for the CIA, then you’ve probably got something closer to high concept. Basically anything that can be summarized in one sentence that someone would want to make a movie out of would be high concept.
That’s how I define it. People use words in different ways all the time. But there’s my two cents. Hope that helps!