Extraños translates to "strangers". While it might seem like a common word, its use in this context is particularly poignant and impactful.
In the song, Jay Wheeler sings, "Cambiamos ser felices para siempre a por siempre ser dos extraños" (We changed being happy forever to forever being two strangers). This line powerfully conveys the painful transformation of a deep relationship into one of complete unfamiliarity, making it a memorable and emotionally charged word.
Jay Wheeler opens this one by taking back every mean thing he ever said. "Discúlpame las veces en que te dije que te odiaba" ("Forgive me for the times I told you I hated you") is his way of admitting that the anger was never the real feeling, it was just what came out when the love got to be too much. He never quite finds a better sentence for that love than the hook itself: "Te amo, te amo, te amo, te amo" ("I love you, I love you, I love you, I love you"), said over and over because one "I love you" isn't going to cover it.
The rest of the song is him arguing with himself and losing. He won't ask her to stay, "Pero no seré quien te obligue" ("But I won't be the one who forces you"), and he tries the old line about how nobody actually dies of heartbreak before immediately contradicting it: "Pero tu me mataste a mí" ("But you killed me"). By the last chorus he's stripped it down to just "Pero ya no" ("But not anymore") on repeat, and even then he can't quite let go, closing the whole song on "Yo a ti te doy todo" ("I give you everything"), present tense, as if she never left at all.