The earth rotates on a tilted axis. When this axis leans towards the sun, it’s summer in the northern hemisphere and winter in the south. This is reversed as the earth continues on its orbit until the axis becomes tilted away from the sun.
During the solstice, the earth’s axis is tilted at its furthest point from the sun. This means that, for us in the northern hemisphere, the sun is at its lowest point in the sky. It’s also the shortest day of the year - and the longest night.