They provide a snip of code to add to your site that creates a tab at the top of the page for your store. When clicked, the store opens up as an overlay on your site, which means your site visitors never have to leave your page to shop.
What I thought was really nice is that you sell your own products, not someone else's. So you could use this to sell your own software, ebooks, music, and other digital content that you have produced. Or you could use it to sell hard goods, as an alternative to etsy or ebay.