The ".old" extension is typically appended to a backup copy of a file. For example, before an important file is updated, a user may duplicate the file and add the ".old" extension to the copy. This indicates it is the "old" version of the file. If the updated file becomes unusable, the old version can be used to restore the file.
IMPORTANT: In order for the backup file to work correctly, the ".old" extension may need to be removed from the filename.