Amazon is horrible at explaining what things are supposed to do. Every product landing page I've seen from them is confusing, especially all of their cloud services.
Introducing Kindle MatchBook
For thousands of qualifying books, your past, present, and future print-edition purchases will soon allow you to buy the Kindle edition for $2.99, $1.99, $0.99, or free.
I'm not sure how they could have stated that any more clearly.
I'm glad you found it clear and straightforward but a glance through the comments here shows that several other people were also confused.
* Applies to past, present, and future purchases.
Essentially, if you've ever bought print editions of books from Amazon, many of the Kindle versions of those books you previously purchased will be offered specifically to you for free or a small charge below $3 if you're interested in adding the digital version to your collection.
It is an extremely unclear landing page.
>>> For thousands of qualifying books, your past, present, and future print-edition purchases will soon allow you to buy the Kindle edition for $2.99, $1.99, $0.99, or free.
That seems pretty clear to me.