A message that those of us old people who used to get on BBSes back when they were not a message board on the internet but another person's computer that we called up with our modem and a terminal program and entered a command like "ATDT555-1212" which caused the modem to dial and connect.

It meant that you were disconnected. If you intended to log off then it was ok. But if someone picked up the phone, or for who known what reason, sometimes you'd get it in the middle of doing something. Maybe even during an online game where you were in fierce competition with someone and because there was only one telephone line for the BBS whoever got in first had the advantage and when you got cut off often someone else would get on instead and you may not be able to get back on, or if so, until your rivals were done leaving you in a worse position.