.PLAYGROUND File Extension
What is a PLAYGROUND file?
Developer file created by Apple Xcode, an IDE used for creating iOS and OS X apps; saves an interactive work area so that programmers can write and test code in real-time while they develop; contains source code written in Apple's Swift programming language, which was released with iOS 8 and OS X Yosemite.
PLAYGROUND files are used to save testing areas for new source code. Once the developer is happy with results of the code in the playground, the source code can then be promoted to the main project. If you'd like to manually browse the contents of a PLAYGROUND file, right-click it and select Show Package Contents from the drop down menu.
NOTE: Xcode 6 is the first version to support PLAYGROUND files.