A writer hoping to interest a decision maker in the film, television, or publishing industries in a story faces a formidable challenge. The target of their pitch is an extraordinarily busy person and will likely only be able to give the writer fifteen minutes of his or her time, tops. This person's job also requires them to listen to a huge number of story ideas that are inappropriate or flat-out Bad, and often has to listen to the same inappropriate or bad ideas again and again and again.
This means that the writer must be able to sum up the whole essence of the story in a painfully short summary. The ideal is to be able to convey the entire concept in one sentence. For example, science fiction writer Michael Moscoe successfully pitched the idea for his military time travel novel First Dawn to an editor thusly: "It's Tom Clancy meets Jurassic Park."
One of the more famous examples of this was the pitch for the 1981 movie Outland: "It's High Noon in outer space." High concept indeed -- take a classic Western that almost everyone knows the story of even if they've never seen the movie (forsaken by the craven, cowardly townspeople, the sheriff of a Wild West town must stand alone in a showdown against the bad guys) and recycle it as a science fiction movie.
In addition to being cited as a classic example of a sucessful pitch, it is also often used to show exactly what is wrong with most science fiction. In a good SF story, the science is essential to the plot. If by changing the names, the costumes, and a few props you can turn a tale of brave space marines fighting aliens into a tale of cowboys fighting bandits, knights fighting dragons, or cops fighting gangsters, whatever else it may be it ain't science fiction. All the SF elements are revealed to be mere window dressing -- which certainly proved to be the case in Outland.
(This goes for other genres as well. In her essay "From Elfland to Poughkeepsie", Ursula K. Le Guin mischievously turns a scene from a popular fantasy series into an modern-day political thriller simply by changing the place names.)
P.S. Yeah, yeah, guys -- I liked Outland too. I'm just saying.