The twenty-four solar terms divide the ecliptic into equal arcs of fifteen degrees, and this one falls at 0 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.
Chunfen is the spring equinox, and it is the term at which the sun's apparent longitude is zero, the origin from which all the others are measured. Four of the twenty-four mark the equinoxes and solstices and are the astronomically fundamental members of the set; the solstices in particular are the earliest of the twenty-four to be attested, and the full system of twenty-four was assembled around that older core.