Scriptwriting is a fantastic way to frustrate the crap out of you. Even if you conquer everything - plot, characters, theme, etc. you still may completely miss the mark - selling it - if you don't pay attention to the tip of the spear:
The Concept. (warning: I'm going to whip this spear analogy to death)
A high concept is (loosely) defined as being "the elevator pitch" - something you can say in a few sentences between floors of an elevator ride. I say parking lot pitch because it's even harder to contain someone who is searching for their car and anxious to get out of wherever they are - elevators are simple. Unless the person you're pitching is an action hero and can escape through the roof, they're stuck. Parking lots not so much - but I digress.
A pitch can also be the logline (I'll give you examples below)
BULL (new series on CBS): Based on Doctor Phil's early life, a psychologist who is a world-renowned jury analyzer solves crimes every week.
BRAIN DEAD: An alien species invades people's brains in Washington and makes them even more partisan than they are causing even more gridlock in the halls of power.
Columbo: A seemingly bumbling detective who is actually a brilliant crime fighter, solves the HOW DONE IT instead of the WHO DONE IT.
The concept here is to pitch someone quickly and concisely so they request your script.
This is improved by having a HIGH CONCEPT. LINK TERMS TO SCRIPTWIKI!
A High Concept is simple. It is revealed better by example than by endless narrative. The three shows I mention above give a sense of that. But let's try to give a narrative definition and then some more examples:
HIGH CONCEPT: A high concept pitch is a sentence or two of description that invokes either a question (in a good way) or a request to tell more. When "Lethal Weapon" came out (back in the dim days of film) it was easy to pitch: Two cops, one is crazy. Action ensues.
That "crazy" part make you want to know more, right? So then this become "The Tip of the Spear!" Yay - here come the whipping analogy part.
A spear tip is sharp. It's shaped for optimal penetration (no, I'm not going there, guttermind.) A spear would be a stick without the tip so it defines the spear as a specific type of weapon. It's...all right, that's all I got for now. So maybe "whipping to death isn't quite the right phrase - mildly flogging?)
Point is (see what I did there) without the spear tip (the pitch/high concept) the stick might still be a weapon but it wouldn't be as efficient.
Many think that high concept is new but it's been around since the dim days of any entertainment medium. Shakespeare was a high concept machine. Hamlet? "A Danish prince meets his murdered father's ghost and swears revenge against this uncle who killed his father."
Wow. Really? Circa 1600 er, something.