The App Merchandise Tab is provided as a way to sell products using your app. It is applicable for both the HTMl5 Web App and the Native app. This video covers most of the basic functions for setup, with the exception of integration with PayPal which should have been done for you already. In addition this video does not cover some of the other advanced product variation features, which will be covered in a 2nd video.
As always if you have any questions feel free to leave a comment below.