It’s just a PowerShell command that goes through all files (recursively through all subfolders) and consideres just the “.json” ones. $ is just the file name, $.FullName is the entire path of the file.
Make sure your PowerShell window points to the correct folder when you run the command - it should be your project root.
I just compile / run the game in VS and when the window with the error appears, I just switch to the opened PowerShell, run the command, switch back to VS and build / run again.