It doesn't really say a damned thing.
The Mayans created a system of calenders that are super accurate. They predict all kinds of celstial events.
How the Mayans knew what they knew, why the Mayans put the things in there calenders - this is a modern day mystery of a long lost culture.
What really gets people crazy is that the Mayans predicted there calenders far out into the future - then just stopped.
Dec 12 2012 is the last day on the Mayan calender.
There is no prediction for that day. It is just the last day the Mayans bothered to predict out to.
It is exactly the sort of thing that causes all the usual people to come out of there usual places and start saying there usual things.