goodroi - 7:01 pm on Dec 27, 2012 (gmt 0)

It means what it says. You had a big traffic change. Big traffic changes tend to be more common around special days like Christmas, Mother's Day, Super Bowl Sunday, etc.

You should look at your analytics for more details to see what went up or down. Look at what urls, keywords and conversion are impacted before you decide to celebrate or cry.