Answer:
Main details largely are about an idea or new concept that's noticeably important to the media it's in. An example would be if a book mentioned that a characters' father went missing and they then go to look for him, this detail is a main one as it's heavily important to the story. A supporting detail might add to a main, but not have as much importance, such as the character with the missing dad saying what their father's name was, while it adds information though the main focus is still that the dad is missing and they're trying to find him.