What does it mean?

Hi. I have a adsl connection with Alcatel Speedtouch usb modem. running on rh 7.3 with SMP kernel. I have no problem on connection. But after a period of time the connection breaks and i read in /var/log/messages “Jun 22 14:53:00 XXXXX pppd[xxxx]: Terminating on signal 15.” and “Connection terminated.” what does it mean??