Apple's Swift blog:
Xcode 6.1 introduces yet another way to experiment with Swift in the form of an interactive Read Eval Print Loop, or REPL. Developers familiar with interpreted languages will feel comfortable in this command-line environment, and even experienced developers will find a few unique features. To get started, launch Terminal.app (found in /Applications/Utilities) and type “swift” at the prompt in OS X Yosemite, or “xcrun swift” in OS X Mavericks. You’ll then be in the Swift REPL.
All you need to do is type Swift statements and the REPL will immediately execute your code.
More information and some examples in the linked post.