Food is basically just pre-made batter, eggs, potatoes, and processed meat; all of which holds well and only requires limited refrigeration. Staff is pretty basic crew: Cooks and customers can order directly at the register if waiter isn't available.
Add to that a culture of staying open at all costs and there you go.
2) it's measuring basic availability of resources like water, electricity, natural gas, but also availability of food items like eggs, meat, bread. Plus labor.
And 3) in extreme cases, whether the (relatively uniform) building still stands and is occupied/usable.
The Waffle House storm center.[2]
They have an emergency menu mode. Only a few items are available, and those are all ones which can be prepared rapidly in bulk.
Waffle House locations deliberately do not use any systems which require continuous Internet connectivity.
[1] https://www.inc.com/jessica-stillman/waffle-house-stay-open-...