A supermoon is simply this: a full moon when it is the closest the moon will get to Earth throughout its orbit, and sits just on the horizon. It appears to be larger than normal because of this illusion. However, it is not. But, it is still an amazing sight to see.
This year the supermoon appears tonight for much of the planet; the only time all 2017. The next supermoon will be on New Years Day, 2018. If you have never seen one, mark your calendars.