This is a very short document, no more than a few sentences, which answers the question, "What is your game about?" See Chapter 5 for more details and examples. It may seem strange to devote an entire document to this brief topic, but it's important to do so.
Like a company's mission statement, it's useful to print this up and post it around the office so everyone has a visible reminder of what they're trying to create. It should also be maintained in a prominent place on the organization's internal Web.
If the high concept changes while the game is in development, that's a major event that should receive a lot of attention. If this happens, make sure to update this document and post new copies where everyone can be reminded how the basic course of the project has changed.