The summer solstice is an astronomical term regarding the position of the sun in relation to the celestial equator. At the time of the summer solstice, Earth is at a point in its orbit where one hemisphere is most tilted towards the sun, causing the sun to appear at 23.45 degrees above the celestial equator, thus making its highest path across the sky. The summer solstice is the day of the year with the longest daylight period and hence the shortest night. This day usually occurs on June 21/June 22 in the northern hemisphere and on December 21/December 22 in the southern hemisphere. The actual date changes due to differences between the calendar year and the tropical year.
At the point of the summer solstice in the Northern Hemisphere the sun appears to be directly overhead at midday along an imaginary line on the Northern Hemisphere at latitude 23.45 deg N known as the Tropic of Cancer. At the summer solstice in the Southern Hemisphere the sun appears directly overhead at midday along an imaginary line 23.45 deg S known as the Tropic of Capricorn. These two lines were so-called because, in ancient times when the first Western astrological charts were set, the sun rose in these constellations at these times.
These lines mark the southern and northern most points where the sun can appear to be directly overhead to an Earth-based observer, and encompass the tropical region of the earth's surface.
Info taken from Wikipedia