Great question — answering as the UphillTech team:
This refers to their difference in perception — the baby can hear and they can't — growing up, she will perceive the world in a different way to them. Later in the movie, Alejandro will talk of "I understand different worlds" — as in his situation, he and his partner could hear, and their child was deaf.
Full answer by Hesperus: https://movies.stackexchange.com/a/115823 (CC BY-SA 4.0).
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