I remember asking myself this question when I began my writing journey. In fact, if you go back to some of my early posts, you can probably find it. But don't do that. Don't! There are many embarrassing things back there. Things that I don't want to relive and only keep myself from deleting them because I know they will incite much hilarity one day.
Today is not that day.
Anyway, the point is, high concept is one of those terms that agents and editors toss around, and every writer at some time in their career has no idea what it means except that it's the coveted grail you need to seek if you want a chance at getting published. It's the hook for your novel, the thing that comes long before your writing, your character development, and your plotting.
So what is it?
Publishing professionals often say it's X meets Y. Or it's something known with something new.
Really, examples work best, so I'm going to show you. Fairy tale re-tellings lend very easily to this.
Young woman is held captive by monstrous beast, only to learn he's not actually a monster. (Beauty and the Beast)
Something New: Tell the story from the beast's perspective and set it in modern day New York. (Beastly)
Poor girl with evil step family captures the heart of a prince with the help of a fairy godmother. (Cinderella)
Something New: Now let's make Cinderella a cyborg and set the story in futuristic China. (Cinder)
But your hook doesn't have to be a fairy-tale re-telling.
Known 1: A virtual reality landscape where people live and work in an imaginary world (The Matrix)
Known 2: Insane genius creates wild contest as a way to pass his business to a new proprietor. (Willy Wonka and the Chocolate Factory)
Something New: Now mash those two ideas together and set it in a future landscape where the escapist virtual reality world is built around the 1980's. (Ready Player One)
Known: Middle aged man becomes exhausted with the status quo, seeks a way out. (The main plot point to about eighty-thousand novels)
Something New: Man invents alter ego of himself and starts an underground fight club that evolves into an anarchist movement. (Fight Club)
Is is starting to make sense? Obviously there have been vampire books before, werewolf books, romance novels, travel novels, cookbooks, books about monkeys, and princes, and middle-aged women who cheat on their husbands, who learn they have cancer. You're not going to come up with something completely new, and even if you do, the story will have already been told. The key to achieving high concept is to take those ideas and twist them, either by changing the players, the viewpoint, the place, the manner of telling, the outcome...something different. Everyone's read a teen vampire romance novel, but were the vampires gay? Was the story told in verse? Or did it take place on another planet?