The “Buy” button will only link to Etsy which houses a lot of hand-made products. For example if you yourself have a product you’ve recently made, you can just link it to your Etsy page and the Buy button will be generated in Tumblr, so your customers will have quick access to it as opposed to having to scour your Etsy page for the product listing.
There is also a “View” button that is introduced that will take users to websites such as Kickstarter, Artsy, or DoSomething. Basically this makes link sharing to cool stuff you find online much easier than before, so users who are interested in what you have posted will only have to click the “View” button to find out more about it.
These changes have already been implemented so if you’re an avid Tumblr user or viewer, you should be able to view/use them in future posts.