What is polyfill?

It's the thick paste you use on walls to smooth over cracks.

It's also when you re-implement native functionality from newer browsers in JavaScript for the benefit of older browsers - see for example the Array.prototype.map polyfill.

> Explanations are sparse

I prefer the term concise - no need to dredge through unnecessary waffle to figure out what something does.

> useful examples rare

Possibly. On most sites I tend towards going straight to the examples because the explanations are waffle, but don't need to with MDN.

> there's an assumption of underlying familiarity

That'll be because it's mostly a reference.

There is a JavaScript Guide which appears very comprehensive, but looking at a few pages it does a horrific job of explaining stuff and has some terrible examples, so, yeah... that sucks. :/