This means, that you are about to rename a file to a name that already exists in the folder but is not shown in the file list.

Example:

Imagine a folder with .htm files and .txt files. After you change the wildcard filter from "*.*" to ".txt" the file list shows only the .txt files.

Now you replace the extension from .txt to .htm:

A filename "hallo.txt" would be changed to "hallo.htm". If a "hallo.htm" already exists (but is not shown because of the filter *.txt) then 

1) the Start button shows "Start and Delete"

2) The file list shows one or more "D"s

3) if you click the Start button you get a warning message window

which unfortunately is in German, because I used a German version of Delphi to make 1-4a Rename. This will change in the next version. However "Ja" means "Yes" and "Abbrechen" means "Cancel".

If you want to rename the files and skip the "D" files then check "Don't rename  problematic files" which skips all files marked as "D" (double name exists on disk) or "!" (double name exists in list) or "0" (illegal name).