Traffic within your network. So if you have digitized your (G-rated legitimately purchased) media collection, and stream that from a PC to your phone on your home network, they could potentially track that, if they control your router.

As your ISP, they wouldn’t be able to see any of that traffic otherwise.

Every device connected to the network, their Mac addresses, any unencrypted traffic. There are lots of local IoT devices that assume they’re safe behind your router. They could get info on everything you watch on TV if you have a Roku (and probably other ones too), see when you open your fridge, your thermostat