In their essay defining the intentional fallacy, Wimsatt and Beardley argue that using an estimation of an author's intentions in a work of creative writing is a poor and/or unacceptable method of literary criticism.
"We argued that the design or intention of the author is neither available nor desirable as a standard for judging the success of a work of literary art" (Winsatt & Beardsley).
When approaching a work of literature, Wimsatt and Beardsley argue, the fact that an author might have specific thematic and formal intentions when creating a work of literature does not also produce a situation wherein these specific intentions might be used as a means of assessing the value or success of the work. In other words, just because a writer has certain aims does not mean that the work should be judged according to those aims.
This critique of intentionalist criticism is explained to hold true because (1) there is no sure way to ascertain an author's intentions for a work of literature, (2) literature is significantly performative and so should be seen in terms of its effects on the audience/reader and (3) works of literature are subject to interpretation from the "public" and thus might be said to have many potential meanings not intended by the author.
Wimsatt and Beardsley conclude by implicitly arguing for a formalist or semi-objective mode of criticism that seeks to assess the value of a work of literature through an examination of artistic qualities.
To extrapolate a bit for clarity, we can note that the intentional fallacy is often understood to describe a mode of criticism wherein the reader/critic presumes knowledge of an author's creative intentions and uses them to judge the success or failure of a work of literature. (Ex: Did the writer achieve his or her goals in this poem if we presume that the writer intended to produce a work of psychological impressionism expressing the culture-shattering experiences of early 20th century industrial advances and global warfare?)
The intentional fallacy suggests that such an approach, on the one hand, simply presumes too much access to an artists creative thinking and, on the other hand, tends to ignore the nature of written work as such. (What is written is meant to be judged by what is on the page - according to Wimsatt and Beardsley - not by the supposed cogitations and impulses of a creator whose mind is not accessible on the page as the poem, story or novel is accessible in the form of plain text.)
Attention should be paid to what is available, as text, as opposed to what must be surmised. The value of a work should be judged by this text alone and not on a relative scale of achievement of surmised artistic aims.
If you are interested in learning more about "The Intentional Fallacy" and New Criticism, check out this video: