You can actually Google "high concept" and get a fair idea of what it means. Wikipedia has a good article on the subject. Steve Kaire has another.
But I didn't understand the concept of "high concept" until CJ Lyons put it to me like this: take a universal, everyday idea and turn it on its ear with a great twist. (She put it in better terms.)
You can sum up high concept with three earmarks: unique, mass-appeal, briefly summarized. Unique, meaning no one's ever come up with this idea before. Mass-appeal, meaning a large cross-section of people would be able to understand and appreciate your idea. And briefly summarized, meaning you could pitch this to an editor in three sentences with no problem.
Sounds like a tall order, right? But work with it. What makes your book so original? What takes it from a typical "boy-meets-girl" romance to the compelling story it is? What makes it different? Try it out and see if you can pitch your book in such a way that its unique qualities stand out from the legion of romances available to buyers. You might be pleasantly surprised at what you find!