Note annotations put a small icon on the page, that, when clicked, pops up a post-it style note to the left of the page. There are different types of note annotations whose only practical difference is the type of icon used for the popup button: Comment, Key, Note, Help, New Paragraph, Paragraph, Insert.
The textual content of the note can be edited in the Dynamic Annotations Inspector. You can also choose whether a note annotation defaults to opening its popup or not. To close an open note annotation click the small close button at the top right of the popup.