What does m c square mean?

mc2 could mean many things, but I assume you are talking about Einstein's famous equation E=mc2. I won't even pretend to know all the information behind this equation, but my understanding is that it basically calculates the amount of energy that could be spontaneously created by any mass. In the equation, E=mc2 E = Energy M = Mass C = Speed of Light (or 299,792,458 meters/second)