How is the story structured with form, and what is the connection between the form and the meaning of Wide Sargasso Sea?

"Form" is a catch-all term for the elements that comprise a work's structure—the basic order of events (in a narrative text), the use of chapters (or other structuring devices), the conventions associated with the literary genre (a sonnet, for instance, follows a very specific template), and so on. "Meaning" is an even broader topic that potentially encompasses not only the work's themes, but also its cultural context and the effect it has on its readers. In order to talk about the relationship between form and meaning, then, it helps to narrow your focus by asking yourself what is particularly unusual or idiosyncratic about the work's structure.
In the case of Wide Sargasso Sea, Rhys has divided the novel into three very distinct sections, with Antoinette narrating Parts One and Three and the Rochester figure (unnamed in Rhys's novel) narrating Part Two. This three-part structure is not especially unusual, but it is significant in light of the fact that Wide Sargasso Sea is a literary "response" to Charlotte Bronte's Jane Eyre; the latter also breaks (roughly) into three sections, corresponding to Jane's childhood at Lowood, her romance with Rochester at Thornfield, and her time with the Rivers family at Moor House. By following the same basic template, Rhys encourages us to think about the ways in which Antoinette's narrative compares to Jane's.
On that note, one of the most striking things about the structure of Wide Sargasso Sea is that it ends almost exactly as it began: Antoinette's childhood home of Coulibri burns down early in the novel, and in the work's final pages she herself is dreaming about burning down Thornfield. This circular structure underscores, in part, how little control the novel's characters have over their own fate; after witnessing her mother's disastrous marriage and descent into madness, Antoinette grows up to experience precisely the same things. We sense, in other words, that Antoinette is destined to relive the same tragedies over and over again no matter what she does—as payment for her family's involvement in slavery, because of her lack of power and agency as a woman, and so on. All of this stands in marked contrast, however, to Jane Eyre; like most nineteenth-century novels, Bronte's has a straightforward and linear structure that ends with its protagonist happily married, underscoring her ability to take charge of and improve her own life.Another unusual aspect of Wide Sargasso's Sea's form is Rhys's choice to have the Rochester figure narrate Part Two—that is, the section of the book that purports to explain how and why Antoinette became Jane Eyre's "Bertha." On the face of it, this seems like an odd move, assuming we want to read Rhys's novel as a kind of "defense" of Bertha. It makes sense, however, in the context of Rhys's broader treatment of identity. Even before she goes mad, Antoinette is an elusive figure—including, significantly, to herself. She comments several times, for instance, on the fact that as a white Creole woman, she does not feel at home either among Jamaica's black residents (her family's former slaves) or the more recent wave of white European settlers. In fact, it is in part the ambiguity of Antoinette's identity that the Rochester figure finds so unnerving and which he connects to the broader "mysteriousness" of the Caribbean. By having Rochester narrate what are (arguably) the most important events in the book, then, Rhys ensures that Antoinette will remain somewhat unknowable even to her readers (perhaps as a commentary on the impossibility of truly understanding someone across vast differences of power, gender, and culture).
Again, these are far from the only ways to think about the relationship between form and meaning in Wide Sargasso Sea, but they are likely starting points.