The twenty-four solar terms divide the ecliptic into equal arcs of fifteen degrees, and this one falls at 90 degrees of the sun's apparent longitude. It is a major term, a zhongqi, one of the twelve that fall in the even positions of the twenty-four. The major terms are the ones that anchor the lunar months, and they alternate strictly with the sectional terms, so a zhong is always followed by a jie fifteen degrees later.
The twelve major terms are what make the Chinese calendar work as a lunisolar one. Each ordinary lunar month is expected to contain exactly one major term, and the month takes its number from the term it contains, so the eleventh month is by definition the one holding the winter solstice. When a lunar month falls entirely between two major terms and contains none, it is declared intercalary and repeats the number of the month before it, which is how a thirteenth month is inserted without any fixed rule about when.
Xiazhi is the summer solstice, the sun at 90 degrees and the longest day. Together with the winter solstice it is the oldest determinable point in the Chinese calendar, measurable with a gnomon by anyone who can mark the shortest noon shadow of the year, and the two solstices are attested in Chinese sources long before the full set of twenty-four was assembled. The sanfu, the three periods of greatest summer heat, are counted from Xiazhi rather than from any term of their own.