[Serusers] route to PSTN

[McAvan, Troy-P29897]

Would it be possible for someone to explain what each line means?
if(method=="INVITE")
{
   if(uri =~ "^sip9.*@.*")
   {
        rewritehostport("ipofmediatrix:port");
        if(!t_relay())
        {
            sl_reply_error();
        };
    };
};  

I am not a software guy and it would be great if someone could explain it line by line possibly.

Troy