It's really just a marketing scheme. Basically, they are just saying they will be actively developing lots of free content for the game for one year. Then, an expansion pack will be released (around that time), adding more content.
If you want the further "free content booster packs" then you'd probably have to purchase the expansion pack. Bugfixes would probably be back-ported to the non-expansion pack editions (I hope), but I doubt you would see more original content. The first expansion will be free for everyone who purchases the game before 1.3 (a peace offering for the rough release).
Stardock has a good reputation for support games for the long haul (Gal Civ's got a *small* patch sometime last year, which was several years after its release). This is just their marketing team trying to capitalize it in a - in my opinion - confusing and misleading manner.
Basically... don't worry. Elemental will get patched for free for at least a year, and it's being used to drive sales. That's all there is too it. No subscription (: