Writing with critical focus means presenting ideas in their intellectual and real-world context, rather than as concepts separated from human society and history.
In some respects, critical focus is a matter of how you write sentences. See, for instance, how many times the name of the writer Peter Elbow appears at the beginning of a sentence or paragraph in the "Believing and Doubting Game" summary on this site.
Keeping Elbow in the picture places the idea of these "games" within the ongoing debates about how to teach writing. In these debates, Elbow advocated the "process" approach--a reaction against the "product" approach of the early twentieth century, which had ossified into the mechanical five-paragraph essay form. Believing and doubting are fundamental parts of Elbow's vision of what writing is and how it can be taught.
Critical focus is not only a matter of crediting a writer like Elbow, but of providing a distance between this borrowed idea and your own arguments, allowing you the leverage to--where necessary--distinguish yourself and your own ideas from his or hers. The "Believing and Doubting Game" page is as much about Elbow as it is about his idea because the identity of the original writer gives an idea meaning and visa versa.
If you write about "Negative Capability" without mentioning that the poet John Keats coined the term, you leave out much of what the term/idea means. Once you put "Negative Capability" and John Keats together, your reader slaps his forehead and says, "Ah, it's Romantic, then! It's early ninetheenth-century!"
Ideas mean partly by way of where--and who, what, when, why--they came from and what conversation or debate they've been a part of.
Critical focus helps suggest the biography of a term or idea.