A polyfill is a browser fallback, made in JavaScript, that allows functionality you expect to work in modern browsers to work in older browsers, e.g., to support canvas (an HTML5 feature) in older browsers.

It's sort of an HTML5 technique, since it is used in conjunction with HTML5, but it's not part of HTML5, and you can have polyfills without having HTML5 (for example, to support CSS3 techniques you want).

Here's a good post:

http://remysharp.com/2010/10/08/what-is-a-polyfill/

Here's a comprehensive list of Polyfills and Shims:

https://github.com/Modernizr/Modernizr/wiki/HTML5-Cross-browser-Polyfills