Vacío means "void" or "emptiness". It's a powerful word that describes a profound sense of absence or lack.
In this song, Sebastián Yatra uses vacío to express the deep emotional state he was in before his loved one arrived. He sings, "Mi vida estaba en un vacío" (My life was in a void) and later, "ya no siento ese vacío porque tú llegaste" (I no longer feel that emptiness because you arrived), highlighting the transformative power of love.
In "Quiero Decirte," Sebastián Yatra sings about being so in love that words just don't feel like enough. He's talking to someone who is used to being adored, asking, "¿Cómo quererte diferente?" (How can I love you differently?) when "a ti te quiere tanta gente" (so many people love you). He wants to declare his love but knows he's not the first, singing, "Quiero decirte que te quiero, aunque no sería el primero" (I want to tell you that I love you, although I wouldn't be the first).
Instead of just saying it, he describes how this person completely changed his world. Before, his life was an empty space: "Mi vida estaba en un vacío, pero tú llegaste" (My life was in a void, but you arrived). This person filled that emptiness and made him feel whole. Since words fail him, he decides his feelings will be obvious in another way, promising that what he wants to say "estará escrito en mi mirada" (will be written in my gaze). It's a sweet song about a love so big it's better shown than told.