This dialog is used to create a self signed SSL certificate for use with the FTP Server. The only thing you need to know is that the common name should be the hostname to which your users will be connecting.
For production use you will generally want to get a certificate signed by a known certificate authority so your users can be confident about who they are connecting to. To get a valid certificate from on of these authorities you will need to generate a certificate signing request. At this time Robo-FTP server does not support generating SSL certificate signing requests. You will have to use IIS or OpenSSL to generate a CSR.