The Comments Panel
The Comments Panel allows you to collaborate on your designs by sharing feedback asynchronously or, if you prefer, making notes for yourself as you work.
A blue dot indicates that a Comment is unread. Comments can be be marked as read and unread by clicking the blue/grey dot. You can add Callout markers to indicate specific places in the wireframe where the comments applies or add a link to an alternate version.
You can learn more about comments in Sharing and Reviewing.