This dialog allows you to confirm that you want to compress the selected files. This will only work with files stored on a NTFS formatted drive! It uses the Native Windows API to tell Windows to compress the file 'in place' using the NTFS Compression. This means that you can shrink your files and not move them from where they are at! In fact your users will not even know that you have done anything!
Simply click Accept to compress the selected files - or Cancel to abort the Action.