The supermoon is when the moon is at perigree.  Perigree means the earth is at its closest point along its path around the earth.

Supermoons actually occur several times a year, and the next one will be on December 14th. But, this one is significant because the moon will be the closest it has been to earth since 1948.  It should be the biggest and brightest moon in just under 70 years. Tonight, the moon will be 14 percent larger than when the moon is at the opposite end of its orbit.

Fun fact: Scientists can determine the distance the moon is from earth by pointing a laser at reflective mirrors on the moon, ones left on the moon during Apollo, and two soviet missions.