What is the ‘Resource Root’ designation for in PHPStorm? For that matter, what does PHPStorm consider to be a resource?

I can answer anecdotally that URL paths will be resolved against “resource roots” where applicable. For instance, in HTML <a href="https://stackoverflow.com/questions/24852739/..."> or <img src="https://stackoverflow.com/questions/24852739/..."> elements, relative URLs are looked up against resource roots. When you start typing within the href or src attributes, auto-completion will suggest items in those directories, and URLs which cannot be resolved will helpfully be marked as a warning.