Julie,
MMP is a directory that is created once your store has been accepted to the Marketplace. In that directory is a file called
storedetails.asp, that file contains basic information about your store categoires and products. When a visitor hits any pages in the Marketplace related to your store, the Marketplace calls information from that file to display in the Marketplace. This keeps bandwidth usage to a minimum across the sites and improves reponse time a great deal, much more efficient than if we were to crawl your entire site each time we need fresh data.
Hope that clears thing up.
Chris