It's exactly what you think it is.
What the developers have to say:
“Prison Architect is currently in Alpha, which means we haven't finished it yet - it's full of bugs and glitches and you're not going to get a polished experience if you buy it at the moment. What you will get is early access to a bullfrog-style Prison Management Sim that over 250,000 gamers have already thought is worth a look at.When you buy early access we also guarantee to give you a free copy of the full game once we've finished and you'll also get monthly updates which include new features and bug fixes. But that's not all, you can also pre-order a copy of the PA art book and Soundtrack (like the game, these aren't ready yet, but you'll be the first to get access when they are launched), or buy a pack that includes the whole Introversion back catalogue (we've made some seriously awesome games). Finally, you can immortalise yourself into the game, by naming a prisoner and writing his bio. Everyone that plays PA will see your bio and know that YOU helped to make Prison Architect a reality.
But that's not all, we also want to get your feedback and comments on the game and want you to help us to direct the development process. Tell us what you like and what you don't like and what you want us to work on. We've got a great community of Prison Architect early adopters and we'd love for you to join in - just head over to http://www.introversion.co.uk/prisonarchitect/developer/regi… once you've bought the alpha.”
I was talking to someone yesterday about how I don't like Prison Architect because like most early access games on steam it just rips off the always-free Dwarf Fortress in the hope that unknowing consumers haven't heard of DF, and other people who can't decode the ASCII have pretty pictures to look at.
……But for $6 this is pretty tempting ;D