From the Washington Post:
The impending full moon, coinciding with the moon’s closet approach to Earth in its orbit, has become known as the “super moon.” In proper astronomical terms, this coupling is referred to as the perigee full moon.
Our lunar companion travels in an elliptical orbit, which means that for every lunar cycle, it snuggles in to a position nearest Earth- its perigee, and strays to a position farthest away – its apogee.
The full perigee or ‘super moon’ appears about 14 percent larger and 30 percent brighter than the more distant apogee full moon.
Read more here.