Foucault spends about the first half of "What Is an Author?" explaining (or really, working up to) his own explanation of what the function of an author is. He begins by explaining that an author is separate from what he writes, even though he created it. We should be analyzing texts on their own merit, he asserts, and not constantly paying attention to the text's relationship with who wrote it--which makes us wonder why the author is even useful at all, beyond that old idea of bringing himself immortality through the written word.
So, toward the middle of the piece, Foucault argues that the author's function is to stand as a symbol, or a persona, so that we can gather up all of the works of that author and think of them as one cohesive body of texts.
The author plays a "classificatory function," as Foucault explains, so that we can assign a meaningful category to a group of texts (for example, all the plays and poems written by Shakespeare). This is useful because we need a way to meaningfully compare one author's body of work with another author's. It's also useful because we can "receive" an author's works in "a certain mode" and give it "a certain status" in our culture.
To put that another way, Foucault wants us to think of authors as a tool for organizing and directing our thinking and judgments about individual pieces of text and about groups of texts. The author (or rather, the author's name) gives us a method for categorizing and comparing groups of written texts.
Foucault spends the rest of the essay describing the implications and problems that arise from this designation of the author's function as a means of classifying texts.