I actually discussed this with our account manager yesterday.
If you go into your customer portal you will see a new key for Tableau Prep. This allows you to install the same number of copies of prep as Tableau desktop licenses you have. I was told this access is available for the next two years, at which point it will be withdrawn and you will have to move to SAS to keep the license. You also don't get the free server licenses that you would get with the SAS model.
I believe that Tableau are trying to encourage people to switch from prep to SAS, but best to talk to your account manager if you want to do that.
Rob's explanation is correct. That is for customers who were current on their maintenance are now given access for the time period specified but you will need to use a separate product key (one that starts with TP and not TD) as outlined in this KB article. There's a link within the article that has a short video on how to obtain the Tableau Prep key. Unlike Tableau Desktop in this instance there's basically one key per portal but it's been preconfigured to match the count of Active desktop licenses so in essence folks will be sharing a key. Though similar to Tableau Desktop if someone no longer needs to have Tableau Prep (ie leave the organization) the key should still be de-activated before any uninstall of the software or re-imaging of the machine.: