As earth rotates around the sun, it's always tilted on its axis at a 23.5-degree angle. Because of this tilt, the latitudes of 23.5 degrees north (called the Tropic of Cancer) and 23.5 degrees south (called the Tropic of Capricorn) become important for the summer and the winter solstices. The tilt gives us our seasons, our meteorological changes, our summer gardens. If Earth had no tilt and was just straight up and down, there would never be a winter or a summer.
If you're a Druid, or other observant pagan, it's the holiest moment of the year. Many, many early cultures began calendar keeping after observing both the summer and the winter solstices. The winter solstice got highjacked centuries ago for Christmas, which is why pagans still hold tight to the summer solstice. At Stonehenge, they'll be solemnizing a religious ceremony in robes and crowns of flowers. (It'll be 4:32 pm GMT at Stonehenge when the sun actually stands still, since Stonehenge is five hours ahead of us. The modern Stonehenge Druids are mostly into sunrise observances on June 21st rather than the exact moment the tilting earth stops its tilt and begins its trip back).
My bucket list includes being at Stonehenge at sunrise on June 21st. Just once. Then we can go have tea.