It means that Automattic gets a share of every .blog domain sold or renewed, and gets to enact policies around what .blog domains are available (like, can you register emoji in domain names) and other arbitrary restrictions about what can be hosted on the TLD (for example, Google was also bidding on .blog to restrict it to only Blogger sites -- but Automattic bid against them so you can do whatever you want t with your .blog)

Basically, it's an investment both in an open web and in a return on investment over the long term depending on volume of registrations.