Bandwidth Monitoring - how does it work?

Operating system: Ubuntu
OS version: 20.04

I would like to where traffic comes from/goes to from my host.

I see the Networking → Bandwidth Monitoring choice in Webmin, but can’t find much documentation about it. (I already have a lot of iptables firewall rules that I don’t want to upset.)

What documentation describes how Bandwidth Monitoring collects its data? Thanks.