What is the little arrow pointing to .swift files in my Xcode project?

In my project some of the code files have a small arrow pointing to the swift icon on the lower left corner. I searched Apple's Xcode docs and tried Google searches by text and also by image, but came up empty. Any idea what this is trying to tell me?

I looked at the files in the Finder to see if they were aliases, but they are not.

That little arrow in the icon is Xcode's way of telling you that the file is a reference to a file that is not within the project's folder. When you added the file, you chose the "Reference files in place" action.

Select the file in Xcode. Display the Inspector on the right side of Xcode. If you look at the file's path, it will be to a file that is outside of the project's folder. Compare that to a file that doesn't show the little arrow in the icon.