What is the solar term of summer solstice?

It is one of the "four solar terms". The four solar terms are: double solstice (winter solstice and summer solstice) and double vernal equinox (vernal equinox and autumn equinox).

"To" means "to the end", not "to". What is the ending?

People in ancient China didn't know about the Tropic of Cancer. This "solstice" means that the shadow of the sun is shortened to the head in summer. It is called the summer solstice. Similarly, on the winter solstice, the shadow of the sun extends to the top of the head.

Modern people all know the knowledge of the Tropic of Cancer. It is easy to understand that the summer solstice and the winter solstice are the true north and tropic of Capricorn of the sun, respectively.

The alternate appearance of summer solstice and winter solstice is due to the fact that the rotation axis of the earth has a "tilt angle" with the orbital plane of the earth.