It originally dates from the old days, when everything was dial-up (eg ARPANet, precursor to internet).
It literally means hang-up the phone line, which is why you use
to ensure a prog keeps running when you logout, otherwise it dies, because its still attached to the terminal session, even if you use '&'. '&' only puts it in the background, its still connected to your term session.
These days SIGHUP is in fact usually used to get a prog eg Apache to re-read its config file on the fly ie without an explicit re-start.