Mockup's notes let anyone viewing a mockup highlight a specific area of the mockup and attach a comment to it.
If you're viewing a mockup, the 'Add note' button can be found underneath the 'Comments & Notes' tab.
Clicking the 'Add note' button activates the drawing tool. Clicking and dragging the cursor across the mockup will create your rectangle highlight.
Once you've highlighted the area of the mockup you wanted to highlight, you'll be asked to write your note. Once you're done, click 'Add note'.
Only the project admin or any project collaborators the admin's invited can edit and delete notes.