The MWS Authorisation Token (Auth. Token), along with your Seller ID, is what we use to link your Amazon Seller account to FeedbackExpress.
FeedbackExpress will ask for these when you are setting up your account, when you add a new marketplace or whenever you need re-link a channel. Once this process has been completed, FeedbackExpress will be able to download your products and sales data for you.
You can see how to get your Seller ID and Authorisation Token here.