Much like with the rest of the film, there is much more about this Titanic scene than just the romantic smooch that popular culture loves to reduce it to. For her whole life, Rose has been denied freedom under the restraints of her surroundings. Her own mother and Cal have put her in a cage, metaphorically cutting her wings by expecting her to conform to patriarchal expections without question. So when Jack asks her to step up over the railing and spreads her arms, he is symbolically spreading her wings. He is opening that cage she’s been trapped in, and showing her that she can be free. She can fly… and fly she does.