A good summary about the solstice from National Geographic begins with this:
The summer solstice is a result of the Earth’s north-south axis being tilted 23.5 degrees relative to the sun. The tilt causes different amounts of sunlight to reach different regions of the planet.
Today the North Pole is tipped closer to the sun than on any other day of 2010. The opposite holds true for the Southern Hemisphere, for which today is the winter solstice, the shortest day of the year.