The December Solstice is a special time of the year when the Sun reaches the point farthest south from the Equator. Imagine a big round ball in the sky, which is the Sun, and a big line around the middle of the Earth, which is the Equator. During the December Solstice, the Sun moves to a point in the sky that is as far away from the Equator as it can get! This happens on December 21st or 22nd every year, depending on where you live.
Now, think about being outside during the December Solstice. If you live in the Northern Hemisphere, which is the top part of the Earth, you might notice that the days are very short, and the nights are very long. This is because the Sun is not shining on your part of the Earth for as long as it does during other times of the year. But if you live in the Southern Hemisphere, which is the bottom part of the Earth, it's the opposite! The days are very long, and the nights are very short.
The December Solstice is important for many cultures and religions around the world. In some places, people celebrate it with special festivals and traditions. For example, in ancient times, people in Europe used to light fires and hold feasts during the December Solstice to help bring back the Sun. Today, some people celebrate Christmas and other holidays around this time of year, which is often associated with the winter solstice.
So, basically, the December Solstice is a very special day when the Sun is as far away from the Equator as it can get, and it affects how much daylight and darkness we experience depending on where we live on Earth.