Milagro translates to "miracle". It's a word that evokes wonder and a sense of the extraordinary, making it quite engaging.
In the song, the lyrics state, "Y como el ciego que un milagro curó, Hoy veo por primera vez el sol" (And like the blind man a miracle cured, Today I see the sun for the first time). This powerful imagery uses "milagro" to convey the profound, life-changing impact the beloved has had on the singer, likening their love to a miraculous restoration of sight and joy.
Río Roma built this one around a complaint about language. The chorus piles up everything the singer wants to say, "Te amo por cómo soy cuando estoy contigo" (I love you for how I am when I'm with you), and then admits the phrase will not carry it: "Porque a veces un 'te amo' no dice lo que uno quiere decir" (Because sometimes 'I love you' doesn't say what one wants to say). So he swaps it for a plainer sentence, and that sentence is the title: "Quiero que sepas que eres demasiado importante para mí" (I want you to know that you are too important to me).
What fills the rest is specific rather than grand. He compares himself to a blind man cured by a miracle, "Hoy veo por primera vez el sol" (Today I see the sun for the first time), then corrects the record on who did the finding: "Siempre te he buscado y fuiste tú quien me encontró" (I have always looked for you and it was you who found me). The best bit is the least romantic one, where the two of them turn out to be badly matched on temperament, "En las buenas y en las malas, yo me río, tú te enfadas" (In good times and bad, I laugh, you get angry), and the argument gets settled exactly where you would expect: "Y si hay guerra, pues con besos lo arreglamos en la cama" (And if there is war, then with kisses we fix it in bed).