This is an interesting question that has more than one answer. The best and most accurate way to understand Hemingway's title is as a double symbol that also represents the overarching theme of the story. First: The title represents Jig's interior references to "hills like white elephants," thus pointing out the main meaning and overarching theme. In other words, that Jig speaks of "white elephants" is a central motif and a central theme of the story that is pointed to and emphasized by the title; thus "white elephants" is the key titular phrase that unlocks the deepest meaning of the story.
Second: "Hills like white elephants" is a double symbol: it symbolizes two things, one of which also symbolizes the other thing. You might think of this double-compound symbol as one umbrella with two people sharing it.
To begin with, "white elephants" are a symbol for that which is holy and sacred. White elephants are a rare kind of elephant, which are not Albinos, that are held sacred in some countries, like India, and in some religions, like Hinduism and Buddhism. Thus the "hills like white elephants" represent a natural monument (hills) that is sacred and powerfully good.
To go one step further into the second symbol, the "hills like white elephants"--now tagged symbolically as sacred and good--symbolize Jig's pregnancy, the natural monumental event that is also an obstacle to advancing along life's path, just as hills can be obstacles on journeys. Now we have two closely related symbols underneath the representative "umbrella" of the title.
Therefore, Hemingway uses the title "Hills Like White Elephants" to point out the deepest meaning of the story; to symbolize the sacred nature of propagating children; to symbolize the pregnancy that is being debated by the American man and Jig.