You can now build and test iOS apps for free (Ahem… after you buy a Mac). Previously to be able to build and test iOS apps you had to pay to join the Apple Developer Program. For $99 a year you were able to download Xcode, gain access to developer resources, and publish your app to the App Store. Now you can enroll in the Developer Program for free, build and test your app, and only pay the $99 if you decide to distribute the app on the App Store.
This is a great way to play with Swift and iOS development to decide if it’s really what you want to do.
You can find more about the developer program here: https://developer.apple.com/programs/how-it-works/