On the night of a "supermoon," the moon appears larger and brighter than any other time. The moon doesn't change, but the way it looks from Earth does.
Since the moon's orbit around the Earth is egg-shaped, sometimes during the month-long lunar cycle the moon is at its shortest distance from Earth; other times it's farthest away. If the moon is full at the same time it's very close to Earth, you see a supermoon.
A supermoon can appear up to 14 percent larger and 30 percent brighter than a full moon.
If you hear a supermoon is coming up, and the sky looks clear, try to get outside to see it. You might have to wait more than a year until the next time. They happen about once every 14 months.