You’ve been running a long rsync job or a Python script on a remote server only to watch it die the moment your SSH session drops, and now you need to understand nohup, screen, tmux, and systemd to stop that from ever happening again.
You logged out for a second. Maybe your VPN dropped. Maybe your laptop lid closed. Either way, that 4-hour database export you were running is gone, and you’re starting from zero.
This happens because Linux ties every process you start in a terminal to that terminal’s session, and when the session ends, the kernel sends a SIGHUP (hangup signal) to everything running inside it, and your processes receive it and exit, by default.