Q:

What is a sub-resource?

According to MDN document.readyState

Returns "loading" while the document is loading, "interactive" once it
  is finished parsing but still loading sub-resources, and "complete"
  once it has loaded.

What specifically constitutes a sub-resource? Is a sub-resource limited to other link elements to external resources?  
(I tried google and MDN and did not find a definition of sub-resource for the web context)

A:

The MDN documentation excerpt you cited is coming from the HTML5 specification.
Note that in order to be consistent, the term sub-resources should be worded subresources (in the original spec. and in MDN).
Even if there is no formal definition of subresources in the HTML5 specification, the term is used later in a way such that you can understand the intended meaning.

A resource's critical subresources are those that the resource needs to have available to be correctly processed.

To answer your question a subresource is everything that has to be fetched after the html has been parsed like scripts, stylesheets, images, videos, ....

