Rift DLC is additional quests that an NPC can give you, but its Subscribestar exclusive enemies. For now, it seems like they are pretty straight forward, just read the hints the Rift contract gives you, find the location and do the enemy encounter.
It in Finchy's subscribestar! 5 dollars a month for the cheapest tier and you get access. The subscribestar is really for playing early versions of the game that are a little buggy and half complete. And for the Rift DLC too. Its going to be content that is roughly a month ahead of versions posted here.