The description mentions buffer terminals for storing extra items from belts, but I don't see any buffer terminals in the game or any of the element descriptions in Factoriopedia. So what are those buffer terminals, and how do I get one?

If you set up a terminal with the "router default" signal to a positive value (has a checkmark over the router's arrow thingie), then extra items on the belt which have nowhere else to go will go to that terminal, which helps prevent jams from extra items. If the signal is only slightly positive (say 20), it won't propagate very far in the network, but if it's more positive (say 10000) then it will propagate much farther.

It would be interesting to make a terminal which buffers some number of items when they aren't in demand, and releases them when they are in demand. Currently that's not supported in the mod, and it's a fair bit of work so I probably won't do it just yet.

In my Py game where I'm using this mod, I usually make a similar functionality by splitting the network. Basically I have (network1 <=> warehouse <=> network2) where the warehouse is connected to both networks (with one terminal for each network). I know which things network1 produces that network2 needs, and I manually enter some amount of them to buffer on the network1-side terminal, and vice-versa. That also reduces clutter in the signaling, because things that are only produced/consumed in network2 won't be on the network1-side signals at all, and vice-versa.

I see, thanks for info!