Someone is going to come and explain better than I can but a quick, probably confusing, answer is it's something you do from the app. On the AMs app you click on the 'AMs Receipts' area and there will be items you can buy pretty much anywhere and get bonus AMs for. You just scan your receipt. As an example, I buy Siggis yogurt and for this I will get 15 bonus AMs until Dec. 13. Plus, for this two week period, there is a bonus of 20 AMs for scanning one receipt.
What I like about this is the fact you can buy anywhere, so when I buy my bagels at Costco, I can use that as one of my receipts and get the bonus AMs.
The items change every other week but there are plenty of repeats. The Siggis has been on since I started.
Hope this helps a bit but I know others will help a lot more with explaining.