Derretí is the past tense of the verb derretir, which literally means "to melt", like ice cream on a hot day. It's a fantastic and evocative word you don't hear every day.
In the song, Fonseca uses it metaphorically to express being completely overcome by emotion. He sings, "Con esa mirada yo me derretí" (With that look, I melted), perfectly capturing that powerful, heart-stopping moment of falling in love.
This song by Colombian artist Fonseca is a huge, open-hearted declaration of love for a lifetime. He starts by admitting he's not perfect, saying, "Puedo no ser tu héroe todos los días / Soy un aprendiz todavía" (I may not be your hero every day / I'm still a learner). He knows he's not invincible, but he makes one very serious promise: when it comes to protecting his partner, "te juro que sí soy el mejor" (I swear I'm the best).
This isn't just a promise for the future; it's about how much he needs her right now. Her presence makes everything better, as he sings, "Si tú me abrazas to' lo malo se me olvida" (If you hug me, I forget all the bad things). He even says she saved him. The song feels incredibly personal, especially when he mentions her by name: "Ay, sin ti, Mariana, ¿qué sería de mí?" (Oh, without you, Mariana, what would become of me?). It's a straightforward, beautiful promise to love someone forever.