Office add-ins are extensions that you can download from the Office store that add functionality to Project. When Project Professional 2013 was released, we were introduced to Apps. Users of Project Standard/Pro 2013 will soon see a name change. My Apps will be renamed to My Add-ins. Project 2016 refers Add-ins, so no changes there.
Office add-ins now have full Read/Write access to the project you are working on.
Here’s a helpful link from Microsoft that explains how to get Add-ins for Project.
Let us know what your favorite Add-ins are: Contact us