In the most basic sense, high-concept means you can summarize your story in a one-sentence pitch, which clearly identifies how your story differs from others of the same genre. For instance, the Hunger Games is high-concept: A young girl replaces her sister as a contestant in the Hunger Games, a nationally-televised event in which teenagers are forced to kill each other. High-concept is easier to pull off with fantasy/dystopian/sci-fi, but it's also possible in contemporary, as with Jay Asher's Thirteen Reasons Why. On the other hand, low-concept novels rely upon character development and stylistic elements to stand out among the myriad of books published each year. If you're writing a book about a woman caught between a lifeless marriage and a new, passionate romance, the execution better be damn good and stylistically unique, because otherwise there's nothing to distinguish it from other books with the same premise.
"High-concept" and "low-concept" change based on what's already out there. While a love triangle between a human girl and two hot, mysterious, otherworldly boys might once have been high-concept, these days it's been done so many times you'd have to come up with a completely different take on the story through execution. This scenario is now low-concept.
One of the reasons I think I'm a high-concept writer is because I start with that one sentence pitch. Many writers begin their stories with characters or scenes, and they build from there, structuring their entire story around one initial stroke of inspiration. I come up with an elevator pitch, then formulate characters to fit the story. I don't think either way is better, although coming up with a one sentence summary from the start certainly makes query writing easier.
I talked about this with my critique group the other day and it's fascinating to see how we all think differently. My stories start out like this: A pair of conjoined twins living in rural India are believed to be the reincarnation of the Hindi god Ganesh, but when one twin dies, the other must deal with the loss of her sister and the villagers' sudden hostility. With this premise in mind, I sculpt characters and refine plot points/themes. However, the other members of my group often started out with a specific character, or a conversation between two characters, and then went from there. Although there are no hard and fast rules, I think the way you initially imagine your stories affects whether or not they're high-concept or low-concept.
I love both high-concept and low-concept books....I actually envy people who can write a good low-concept story, because I think it's often harder than a high-concept one. What about you guys? Are you a high-concept writer or a low-concept writer? How do you start your stories?