Seafile is file synchronisation and file sharing software. Before it works on other devices and in Roundcube, you need to log into the web interface (https://cloud.yourdommain.com/seafile) once. Login with your full email address and email password.
You can use Seafile to synchronise files (Seafile client), to mount the external storage directly (SeaDrive) and to access your files via a web browser. Your Seafile files will also be accessible from within Roundcube webmail.
Most users need the Seafile client. See  for instructions. In the "Server" field, you need to fill in "https://cloud.yourdomain.com/seafile". Enter your main email address and password.