The Fedora Playground Repository would effectively be a staging area for new packages before they are up to the standards of being added to the main Fedora repository. Fedora Playground packages would be packages destined for inclusion into the main archive along with other packages that are never going to make it there. Stability wouldn't be guaranteed by this archive and it would continue to meet Fedora's strict requirements for packages needing to be free software and cannot contain proprietary or patented software.
Packages for the Fedora Playground would be built using Fedora COPR, the PPA-like system that's been generating a lot of attention in the past few months for more easily distributing software for Fedora users and is similar to Ubuntu's Launchpad PPAs.
Overall this is an interesting idea and should allow for new open-source software to get to Fedora users more quickly. More information on this repository proposal can be found via the Fedora Project Wiki.
Fedora 21 is planned for release around October of this year and will be home to a lot of new features.