This is a term much heard in connection with movies, but literary agents also use it:

I love high-concept books. A lot of the books I read and represent are high concept and get a lot of film interest. I define high concept as a premise that can be boiled down into one sentence and sets it apart from other stories by its unique hook or angle. –Paige Wheeler, literary agent

One OED definition is
high concept: adj. Of a film, television show, etc.: based on a striking and easily communicable plot or idea; (also) relating to or characteristic of such productions.

Sometimes “high concept” is used in a derogatory sense to refer to a dumbed-down story that will appeal to a mass audience:

2003 Independent 30 Apr. I. 3/1 The so-called ‘high concept’ pitch has long been considered a byword for crass commercialism in the film world, a symptom of studio executives’ reluctance to focus on anything for more than a minute. –OED illustration

According to screenwriter Steve Kaire, it’s the premise that makes a story idea “high concept.”

The premise or logline is the core of High Concept. My comprehensive definition of High Concept is comprised of five requirements, each of which is mandatory.

Kaire’s five requirements are:

an original premise
mass audience appeal
story-specific pitch
easy-to-see story potential
a pitch no longer than three sentences

Alexis Niki at AbsoluteWrite adds the requirement of “an empathetic hero who is dealing with a BIG problem.”

James Bonnet at scriptforsale thinks that a high concept story should have a great title. Certainly the following movie blockbusters have titles that suggest the theme of their stories:

Star Wars
Jurassic Park

Perhaps the most important ingredient in a “high concept” work is the same one that every good story requires: a main character with whom the reader or viewer can identify. The chief difference between a “high concept” story and a “regular” story is that the main character’s success or failure will have huge consequences for others.