The Winter solstice is the moment during the year when the path of the Sun in the sky is farthest south in the Northern Hemisphere. It brings us Yule on this the shortest day of the year. Those pagan beliefs such as the Holly King transferring his power to the Oak King usher us into a new season of light ...
While it's all mystic and steeped in wonder and pagan magick the day is mostly a mechanism for tracking the seasons by our ancestors.
Regardless, enjoy the advent of Winter and have a cool Yule with it's advent of renewal just in time for the celebration of Christmas.