Where can I find a definitive answer for what `*/` means in Bash or Zsh?

I was trying to look into some reference from O'Reilly about Unix and Bash about the meaning of */ but couldn't find any. We can echo */ and see all the directories. It seems like it means all "directories" only, while * means "all files and directories", but for some reason, many users seem to not know it, and books don't mention it either. Is there a definitive source that talks about what */ means and perhaps its variations?