Dani Fernández opens this one with a confession: "Ahora sí que siento vértigo" (Now I really feel vertigo). Something huge is coming, "Lo que viene es tan inmenso" (What's coming is so immense), and his biggest worry is letting the other person down: "Que ojalá que no te falle" (I hope I don't fail you). It reads like a love song to someone who has just arrived in his life, and the lyrics keep hinting that this someone is very small. Look at "¿Cómo algo tan pequeño puede venir a salvarme?" (How can something so small come to save me?) and "Quiero verte observar con la mirada llena de curiosidad" (I want to watch you observe, with eyes full of curiosity). That curious little gaze sounds a lot like a baby taking in the world.
The chorus is a promise, and it is sweet without getting sticky. He's dying to tell this person "Cosas que no he dicho a nadie" (Things I've never told anyone), and whenever something falls apart, "Solo tienes que avisar" (You just have to let me know). Watch the little wordplay too: first "me muero por contarte" (I'm dying to tell you), then "me muero por cantarte" (I'm dying to sing to you). One letter changes and telling becomes singing.
Learner tip: morirse por + infinitive is everyday Spanish for being desperate to do something, and avisar means to give someone a heads-up, not to advise them.