To participate in Advantage, you must have distribution rights, and your book must have a scannable barcode of the ISBN. You get acim of the retail price of your book, Amazon gets 55%. The amount you are paid is based on the retail price you set for the book, and is not affected if Amazon chooses to discount the price at which they sell the book. If you set a list price of $20 for your book, Amazon will pay you $9 for each copy sold.
There are authors who do not like the fact that Amazon demands a 55% discount. They believe that Amazon should receive a discount comparable to what other book stores would receive, such as 40%. However, Amazon.com is not your average book store.
If you sell to Amazon through a wholesaler or distributor, you will have to give at least a 55% discount and probably more. And, it will take longer to get the money from anyone else. The wholesaler or distributor would pay some time after they were paid by Amazon, so it would probably take at least 30 days longer to get the money. And my experience when I have sold directly to book stores is that is often takes several months to get paid. I have never had to so much as send an invoice or an email to get paid, on time, by Amazon. Plus, no book store can give you the worldwide visibility you get on Amazon.com.
When you are in the Advantage program, you ship books to Amazon and they fill orders to customers. Once your application is approved you will receive your first order from Amazon, usually for one or two books. When those books sell, Amazon will order more. Expect that your first orders will be for one or two books, but as sales pick up they will order more at a time.