The solstice happens when the sun is the farthest it gets from the equator. It is either over the Tropic of Cancer (summer in the Northern Hemisphere) or the Tropic of Capricorn (winter in the Northern Hemisphere) at its highest point. It stops moving north or south on the solstices.
On the equinoxes (spring and fall), the sun is over the equator at its highest point.
It used to kill me as a kid that winter didn't start until late in December. Where I lived we usually got snow early in November. Or often on Halloween.
Now where I live, it's not quite so bad, but we did have below zero (Fahrenheit) temperatures and snow in early December.
Some people try to ascribe mythical qualities to equinoxes and solstices. But they are simply a result of the tilt of Earth's axis.
Starting today, the time of daylight will get longer until the summer solstice in June.