Dueño literally translates to "owner." While this might sound intense in English, in many romantic Spanish songs, it's used as a poetic way to express deep passion and exclusivity.
In "Un Sueño," the singer wakes from a nightmare where his love is with someone else. Relieved to find it was just a dream, he sings, "Hoy soy tu único dueño" (Today, I am your only owner), passionately declaring that he is the one and only in her life. It's a dramatic and memorable word choice that perfectly captures the song's emotional rollercoaster.
Ever had a nightmare so bad you woke up crying? That's exactly where this song starts. The singer tells us, "Hoy en la mañana desperté llorando" (This morning I woke up crying) because he had a terrible dream. In it, he sees his girlfriend looking beautiful in a white dress he once gave her. But she's walking with another man who is trying to marry her! Just as this other guy leans in for a kiss, the singer is saved. He believes, "Será por eso que Dios me despertó" (That must be why God woke me up).
Luckily, it was all just a bad dream. The relief is huge when he wakes up and finds his girlfriend right there next to him. He exclaims, "¡Qué bueno que fue un sueño!" (How great that it was a dream!). The song is a dramatic expression of jealousy and the fear of losing someone you love. He ends by declaring, "Hoy soy tu único dueño y así quiero vivir" (Today I am your only owner and that's how I want to live), which is a very intense way of saying he never wants to let her go.