Wikipedia: http://en.wikipedia.org/wiki/High-_and_low-context_cultures

Short version, in a 'high context culture', things can be left unsaid and the cultural background will explain effectively to people within that culture (and less effectively to people outside that culture).

A Texan (the American south is high context) can say quite a bit with 'yep...' and letting the culture explain the rest of the meaning.

In a low context culture, less emphasis is placed on single word choices (though they can still be important) and more explanation of the entire context of the communication is done.

High context is vague. As in, you're expected to already have a good idea of the context.