In the immortal words of Paul Vasquez, what does it mean? Well, we know that TERA will have a subscription-based business model, but Digital River's press release points to a bit more than that: "Digital River will manage in-game commerce and launch an online store to support North American sales of En Masse's soon-to-be-released flagship action MMO, TERA."
It's almost standard these days for MMOs to offer microtransactions even on top of a subscription fee, so while nothing has been officially announced, the mention of in-game commerce looks like our first indication that TERA may be going in that direction. The agreement will run through 2014.
[Edit: Senior Community Manager of En Masse Entertainment has responded to some of the player concerns: "A few folks have expressed concerns that TERA users are going to have to pay micro-transaction fees in order to play TERA; that's not the case. We'd like to clarify that TERA is a subscription-based game, not a micro-transaction-based game. As gamers ourselves, we believe that our fellow players shouldn't be required to pay micro-transaction fees in order to have a great gaming experience."]