This option helps you to have a backup of the destination file before over writing it with the moving file. The original destinaion file will be replaced by the extension provided with the command.
In above example, we have moved 'file1' into 'file2'. After passing the command 'mv ?suffix=.txt file1 file2', 'file1' will be replaced by 'file2'. While, already existing file 'file2' will be replaced with the extension 'file2.txt'.
Extension file will denote that it is a backup file.