A note is an entry in the change-log, without a link to a subject. Adding notes is only possible for registered maintainers of the logbook. If your are one, you were told what to do (click on the yellow "sticky note" icon) . If you are not a registered maintainer, you can't add notes.
The first line of the text you enter will be used as the title of the note. The rest will be used as the description.