A service is a process or application which is running in the background, either doing some predefined task or waiting for some event. https://lym.readthedocs.io/en/latest/services.html

Service files are in /etc/systemd/system/ and are named SOMETHING.service.

This one runs iPerf3 server so you can do bandwidth tests whenever you want.

# /etc/systemd/system/iperf.service
[Unit]
Description=iperf3 server
After=syslog.target network.target auditd.service
 
[Service]
Type=simple
Restart=on-failure
RestartSec=1
RemainAfterExit=yes
ExecStart=/usr/bin/iperf3 --server --verbose --logfile /var/log/iperf_server.log --pidfile /var/run/iperf.pid
 
[Install]
WantedBy=multi-user.target

After you edit the service file you have to reload it with

sudo systemctl daemon-reload

… and then restart the service with

sudo service iperf restart

…and then might want to check it's status with service iperf status.