Complex sentence: Saros uses an XMPP/Jabber server for session initiation and will also use it for all its internal protocol communication if some participants cannot be reached directly via IP (because their computer is behind a firewall or NAT).
Split into simpler sentences: Saros uses an XMPP/Jabber server for session initiation. It will also use XMPP for subsequent communication if and insofar as some participants cannot be reached directly via IP (typically because their computer is behind a firewall or NAT).