This feature aims to encourage shoppers to buy co-ordinating items/accessories in addition to products they're purchasing from your store. When a visitor adds an item to their basket, the feature displays a screen confirming that the item has been added, along with a list of selected products that the customer may also be interested in purchasing. Shoppers can directly add any of the items to their basket via the screen, or click on the links to view the relevant product pages if they'd like further information. You can choose which products you'd like to up-sell, per item, via your store's admin system.
Please note that this functionality is not available on standalone mobile sites.