Supermoon is basically a full or new moon that occurs while the moon is at the perigee of its orbit. Perigee is the part of the moon's orbit when the moon is closest to Earth. As opposed to apogee when its is farthest from Earth.
When Supermoon occurs, the Earth, Sun, and Moon is almost aligned in a straight line.
Supermoon does not cause earthquakes (as some people have suggested). I wrote a Hub all about Supermoon here:
Unfortunately, I did not see it since it was cloudy and rainy. But it is supposed to look 14% larger than usual, even though it will be only just a little bit closer to the earth than normally this year. It is true that March 19's supermoon will be the closest that the moon will be to earth since 1992