Google super-charges Reader and turns it into an Interwebs slideshow
It seemingly has nothing to do with your own RSS feeds, either. In fact, other than the ability to 'share' or 'like' items, it really doesn't resemble Google Reader at all. Presumably, Google Reader Play builds upon the 'magic' sorting and 'exploration' that the Google Reader team implemented back in October 2009; it's probably just a new UI over the same functionality.
The UI is nice though, and I have to admit that the 'exploration' feature in Google Reader is something I often use. If you're really lazy, or if you just want to watch something hands-free while you eat your breakfast cereal, there's even an option for an automatic slideshow. Give Reader Play a go, or go read the official announcement if you want more info.