Add to Menu provides a quick link in your Joomla! administrator pages to quickly add a menu item link for the article or page that you are viewing.
So you need to create a new article and a link in your menu to that article?
This normally takes a lot of steps; creating the article, going back to the menu manager, creating a menu item there, selecting the correct page type, etc, etc.
With Add to Menu this is done easily and quickly: Simply create the article and apply it. Now, whilst still in the article edit page, you can click on 'Add to Menu'. Select the menu you want to link to and voila! You are done.