Aurora boreal is the Spanish term for "aurora borealis," more commonly known as the "northern lights." It's a stunningly beautiful and rare phrase to find in a song.
Arcangel uses this as a poetic metaphor to describe a woman's beauty. He compares her to this breathtaking natural event, calling her a "fenómeno en el cielo, difícil de no mirar" (a phenomenon in the sky, hard not to look at), emphasizing that she is a rare, unique, and captivating sight.
Have you ever had a crush on someone you've never even met? That's exactly what Arcangel is singing about in "Hace Mucho Tiempo." He's completely obsessed with a woman he only knows through his dreams and what other people have told him. The chorus repeats this idea: "Hace mucho tiempo que quiero conocerte" (I've wanted to meet you for a long time), saying he's seen her in his dreams and wants her "Solita, exclusiva para mí" (Alone, exclusively for me).
He spends the whole song building her up into this perfect, almost mythical person. He compares her to everything from a Greek monument and the canals of Venice to the Northern Lights in Norway! He even says she's as "Dura como la gran muralla China" (Tough like the Great Wall of China). It's a classic reggaeton track about intense desire, but with a twist. He admits he's never actually seen her in person, but he feels like he knows her because, as he says, "al soñar veo tu rostro" (when I dream I see your face).