The title is a huge focus for most people interpreting this story, in large part because the title is layered into the story in various places. First, we should notice that the title is a simile (a comparison of two or more things using "as" or "like"). This suggests that simile will be involved in the story. The simile is ‘free-floating’ in the title. We don’t really have enough information to understand why it is significant, even though it might bring to mind an image of white hills, maybe with trunk and tusks, possibly charging through an exotic landscape. The title doesn’t really give us a clue that we will be entering a story which is essentially about a man and a woman discussing the future of their unborn child and their own futures.
Let's look at the word, hills. We all have a certain image when we hear this word. Maybe the hills you imagine are green and rolling, or maybe they are brown and dusty. In this case "hills" means "mountains," though "hill" is often used to refer to a geologic formation that isn’t quite as high as a mountain. A hill is any raised mound of earth, or mound of chocolate pudding, or of whatever. As readers and critics of this story often note, a pregnant woman’s stomach can also be consider a hill.
You could also think of "hills" figuratively, as a barrier that isn't easily crossed or overcome. Among other things, this figurative meaning could apply to the inability of the two people to communicate in such a way as to come to an agreement with which they both feel comfortable.
So, now let’s look at what the hills are like. We can think of the literal definition of "white elephant." Since elephants are usually grey, we can assume that white or albino elephants are rare, but they do exist.
According to the Oxford English Dictionary, a white elephant has a more figurative meaning: "A burdensome or costly possession. Also, an object, scheme, etc., considered to be without use or value." It’s easy to see how this could be discussed in context of an unplanned pregnancy that at least one of the parents wants to terminate.
We should also consider whose simile this is. It’s Jig’s. The fact that she speaks the title, in some ways, suggests that the focus of the story is on her.