Snark aside, this is actually a really cool feature. Essentially, when you're running a Twitch stream of the game, you can let your viewers affect your game by using special commands in the Twitch chat that will then translate into things like a horde of zombies appearing or giving you a health boost. This is strictly an optional feature and you can tune how much you want certain interactions occur.

The game clearly has a ways to go, as actually getting a co-op game started is a bit tricky. You'll need to bring up the communities menu and then allow certain friends into a community. Once that is done they'll be able to see your game in a list of available games. 

505 plans to release How to Survive 2 in Steam Early access sometime around Halloween. The current plan is to release new updates to the game every two weeks or so, with specific goals in mind of feedback during each cycle. This is the first foray for 505 into this uncharted territory of early access, but it sounds like they at least have a battle plan for release - which is refreshing compared to what we've seen from other early access titles.