The idea is that you can order items that you're running low on as seamlessly as possible. Just scan the bar code of your half-empty bottle of milk or say "chocolate chips" into the microphone, and that data will be sent to your AmazonFresh account, where you can log in, refine the list, and officially purchase everything you need.
Because that's the big catch: To use Dash you need to use Amazon's same-or-next-day delivery service, AmazonFresh, and to do that, you must live in either San Francisco, Southern California or Seattle (at least for now).
But if you do, it's your lucky day, because the device is completely free. Amazon's game plan with Dash is ultimately to encourage people to buy more stuff from the site. Like the Kindle makes Amazon money because it gets users to buy more books, and the Fire TV will make Amazon money by getting users to buy movies and games, the Dash will likely amp up the number of products that people order through AmazonFresh.
Check out Amazon's (adorable) video:
Disclosure: Jeff Bezos is an investor in Business Insider through his personal investment company Bezos Expeditions.