It's a combination of two main ideas. The Mayan Calendar stops in December of 2012 so as they were oh so accurate, doom sayers claim they foretold the end of times. Now either they were just that smart, or sheer luck, but the day that calendar does stop is the moment in our history our sun and planet cross directly over a galactic medium where the center of the universe, our star, and us (think of a piece of paper) cross from above to below it. It is unknown to any science if nothing will happen, or something could happen. Sky could change color, we may get irradiated into atoms, who knows.
So it's all a play to mess with people or we're fucked...no one truly knows.