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.
Remember:
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.