I want to be able to look at and understand the network traffic coming in and out of my house. I know enough to use Activity Monitor and things like iStats for one computer, but I’m really more interested in understanding/viewing it from the modem level – I have lots of variation in my download/upload speeds with no discernible cause, and 6 devices that could be contributing to the issue.
Can anyone recommend good beginner guides for understanding/tracking network data?
You’ll need a good way to instrument the traffic.
Any Linux system/box can be rigged to do it. For consumer, devices like the Firewalla can be a good way to dip a toe into the water.
Full blown analysis, you’ll want to learn Wireshark and have a good NIC. TBH, if something requires dusting off my WireShark skills, I’m strongly motivated to find another, any other, way to solve the problem instead 
TL;DR - You’ll want an in-line device attached to your WAN connection so you can see the actual traffic in and out. Looking from a client device like your Mac doesn’t have the visibility to see everything before processing or fitering has been applied.
Running the Little Snitch app and all the background info available at the maker’s ObjDev website might also be a good start.
Thank you both. Some good starting points and things to consider.
I know somebody that an issue with their upload/download speeds from their cable modem. Turns out that the line from the provider was chewed up by squirrels. Test each step of your connection - line into your house, cable modem, ethernet cables, and wifi router. Try to narrow down which component is causing the variability. I start by testing everything wired to reduce the variability.
After some internet speeds improvements last year I started to try to learn more about networking. YouTube has a few videos that go over everything in a big picture type way, but often going down to the small home network level wasn’t very helpful. Watching them made me want to set up my own home lab with a Unifi network though. 
One channel I found helpful is called Crosstalk Solutions, but there is a ton of stuff out there.
One thing I had overlooked for 2 years is that my internet plan was faster than my router supported. My old Eero only had a 1 GbE port. My plan was 1.2 Gbps. So I could never get full speed from a wired connection. Finding routers with faster ports isn’t difficult, but the prices go up quite a bit when you get into that area, especially if you want more than one 2.5-10 GbE port.
So now I am on fiber, and wired connections are blazing fast, but almost nothing I own supports Wifi 6e, so everything is still slow…