It's an explanation of most of the stuff which was mentioned in the translation notes. I.e. what a kaichigo is, the shell-matching game from the Heian period, how "shell-matching" is an euphemism for lesbians, etc.
the translation notes says "it is amazing what you can learn from translating porn." I was thinking the same thing, "it's amazing what you can learn from reading porn." I guess I will forever be a student.
