I do not understand the Reusable CSS with mixins lesson

No, that’s just prefixes for CSS. So -webkit- will specifically tell the HTML that if the browser is a mobile browser, then use the webkit property. -moz- will tell the HTML if the browser is A Mozilla Firefox, it will use that. -ms- is the same but for Microsoft. It’s just a prefix for browser compatibility.

Here’s a full list of Vendor Prefixes: