The "Order" in Foucault's "The Order of Things," refers to what according to Foucault was that all periods of history possess certain underlying conditions of truth that constitute what is known as valid scientific discourse. In "The Order of Things," (to be referred to hereafter as "Order,") Foucault searches for the STRUCTURE of knowledge in disciplines such as history, economics, sociology that emerge and develops the notion of episteme, arguing that the conditions of discourse change over time, although withinin given disciplines, such as linguistics, biology or economics, they seem to hold fairly steady parallels. Discursive Power is the disciplinary authority that these fields of study wield through strategic use of reason and logic; but it is a certain KIND of reason and logic that scholars and scientists use that then become the norm for all discussions. This is known as discursive power.
Now, your question, what has all this got to do with literarature -- or more precisely, how can Foucault's notion of discursive power in the "order" be applicable to literary theory is actually very interesting. (The link to literature is through theory).
In order to understand this, we need to clarify what theory means these days in the post poststructuralist world. Originally, a theory meant a generalization about natural or social phenomena that could be borne out through observation and experiments in a repeatable and systematic manner. Once established that a certain theory "works," scholars and theorists of the Modernist era (i.e., the period between early 20th Century through the 1950s) tried, in their arrogance, to make the theories applicable to, and therefore answer, a wide variety of phenomena that the theories were not supposed to do. Critics of Modernist theorists called them, somewhat sarcastically, "Grand Narratives."
Nowadays, theories thrive through pluralism; i.e., at any given time multiple theories try to explain a set of social phenomena, some of them even contradicting each other. Postmodernists thus stay away from "final answers," entertaining instead multiple theoretical explanations. This is especially true of what Foucault used to call the human sciences, of which literature is one.
Foucault's pioneering work has therefore helped us to return to the discursiveness of literary criticism -- i.e., to genuinely practice reason and logic, but without modernist arrogance.