Can somebody please explain what the following means?
(in the installation instructions for the unsubscribe script)

2. chmod unsubscribe.dat to 666 using your FTP software.

Thanks

The chmod command (abbreviated from change mode) is a shell command in Unix and Unix-like environments. When executed, the command can change file system modes / permissions of files and directories.

Most FTP software have a function to change file permissions. You have to change permissions to 666

What FTP software are you using?