Nope, it's not a story about a blushing bride walking down the aisle.
So what is The Woman in White referring to? Well, the most obvious answer is Anne Catherick, the mysterious crazy woman who always dresses in white (because someone once told her she looked pretty wearing white). Walter, our main narrator, frequently declares that Anne drives a huge amount of the story's action. As the "woman in white," Anne has a sort of ripple effect that impacts practically everyone in the novel.
But there's another "woman in white" the title alludes to, and that's Laura Fairlie, the hero's romantic interest and someone who tends to dress in white and pale pastels. The novel frequently ties Laura and Anne together—the two women even swap places for a time. By referencing both Laura and Anne, the title The Woman in White manages to clue us in to one of the book's major themes: identity.