A Quick Note is a hidden note that can be given to any and all fields in the system.  It is activated by right mouse clicking on the info button next to the field:

The following menu appears, select the Edit QuickNote selection:

An input area appears, in which you can type the note:

Clicking on the Save button to save.  From now on the note appears when the mouse moves over the field:

And the system tells you that there is a Quick Note by highlighting the info button.

To remove the note, edit it and delete all the text.