Systemd is the system level init system which can be used to keep services alive and start them at boot. I bet you had to either write the systemd unit by hand or copy it from someone else. pm2 is nice because it will generate that unit for us. During regular operation it will monitor tiddlywiki and node, should they crash pm2 will restart them for us. Then on boot the systemd unit that it generated will start the monitoring up for us.