If high concept were a person it would be a teenager because it's often totally misunderstood. If high concept were a tool it would be a sledgehammer. If high concept were a okay I'll stop now.
So what does high concept mean?
High concept means that a novel/movie/TV show's plot can be described very succinctly in appealing fashion.
Kid wins a golden ticket to a mysterious candy factory? High concept.
Wizard school? High concept.
There's this guy who walks around Dublin for a day and thinks about a lot of things in chapters written in different styles and he goes to a funeral and does some other stuff but otherwise not much happens? Not high concept.
High concept is very often misunderstood because what it sounds like it means and what it actually means are basically completely opposite. It doesn't mean sophisticated (opposite), it doesn't mean cerebral (opposite), it doesn't mean difficult to describe (opposite). And it's very important to know what it means because although high concept is often a term used derogatorily, I am hearing from more and more editors that they want high concept novels, even for literary fiction.
Why? Well, my hunch is that the more media, the more Tweets, the more links we're constantly besieged with, the more readers are drawn to hooks that we can easily understand and digest.
So not only do you need to know what high concept means, you might also want to consider embracing it if you're thinking of a new project. But only if it's true to the story you want to tell.