Ever since I was told by an agent that the work he was looking at was high concept, I've been puzzling out what it means. I've now found a very good blog on the subject which really does explain it clearly.
This is what it says: "Simply put, a high concept is an intriguing idea that can be stated in a few words and is easily understood by all." Worth checking out the whole article which is on http://www.writersstore.com/conquering-the-high-concept
Even if your book is not actually high concept, the article gives a useful set of tools for working out the premise of your novel so that you are clear about where you're going. Plus it can be used to pitch to agents or editors, especially if you are networking and need a simple statement of your book.
What you're doing with this is encapsulating the story. The premise is king, and it doesn't matter whether your book is plot driven or character driven. It doesn't actually matter whether it's high concept either. The purpose of the exercise is to be able to give the storyline in a few well-chosen words.
Incidentally, there are a number of articles on the net about this subject, but I found this one the easiest to grasp.