Help Creating For Loop to Run Functions on Multiple Files

I wrote a function that I want to apply to every file in a folder. I found this resource (https://www.mathworks.com/matlabcentral/answers/1445389-applying-a-single-function-to-many-files-in-one-folder) for creating a loop to apply my function to the files in a folder. The code shared is as follows:

 FolderIn  = 'D:\Your\Folder';
 FolderOut = 'D:\Your\B'
 FileList  = dir(fullfile(Folder, '*.mat’));
 for iFile = 1:numel(FileList)
   File = fullfile(Folder, FileList(iFile).name);
   Data = load(File);
   % Now do what you want with the data
   NewData = Data;
   save(fullfile(FolderOut, FileList(iFile).name), 'NewData', '-struct');
 end

1. I don’t understand what “ D: ” does in their code. In the first two lines. 2. Even when I set the directory to the folder containing the files I want to reference, the “fullfile” function does not run when I give it the file name. 3. Is there a better way to run my function on a bunch of files?

Thank you!