High concept is a short-hand way of describing any writing project designed to leverage vividly off existing projects with which the audience is familiar. I associate it particularly with Hollywood producers, but I've seen it used plenty by people in the publishing industry, too.
"The script is Twilight meets Terminator!" "It's a cyberpunk Cinderella!"
Or, to use as an example a fine piece of space opera recommended to me by a good (somewhat nerdy) friend, "Midshipman's Hope is Horatio Hornblower in space!"
David Feintuch's Seafort Saga (of which Midshipman's Hope is the first book) is a series of space adventures, and it features space adventure tropes: faster than light travel, colony worlds, demented ship's computers, conflict with space aliens, etc. But the real driver of the books is the operation of its main character, Nicholas Seafort, his personality, relationships and above all conscience, through various crises and within the tight and sometimes paradoxical confines of (space) naval regulations. Sound familiar? Nicky Seafort is Horatio Hornblower with a raygun.
You can describe projects after the fact of their creation using high concept; you can also create them that way. I suspect that Feintuch deliberately modeled his series on Forester's.
Homework: describe your existing projects using a high concept approach. Come up with three new project ideas and describe them in high concept.