Roundcube its just web-based mail client like outlook, thunderbird and etc.
So you must install your own mail server (services postfix, dovecot, nginx) before you can use Roundcube to access it.
Postfix - is mail transport service
Dovecot - its IMAP service, who let your client see their mailboxes
Nginx - www server
As I know (but i not use its) iredmail, kolab - is fullstack packages - full mail servers with web based clients and all services.