Augmented reality is what it sounds like: reality, enhanced with interactive digital components. The most commonly used AR applications these days rely on smartphones to showcase the digitally augmented world:

To create your first RealityKit app with Swift, you will need to have updated to MacOS Catalina. Neither of these new frameworks will work on MacOS Mojave or any earlier version. You should also make sure you have the latest version of Xcode by checking the Mac App Store and checking that there are no updates for Xcode that you have not yet installed. Having the latest version of Xcode and MacOS is essential to making sure this app works correctly.