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.
In "Hace Mucho Tiempo", Puerto Rican reggaeton artist Arcangel sounds completely obsessed with a woman he has never even met in real life. The whole track revolves around the idea that he has heard fantastic rumors about her and constantly sees her in his sleep: "Que te vi en mis sueños que ya quiero tenerte" ("I saw you in my dreams, now I want to have you"). Because he has not actually spent time with her, his imagination goes completely wild, stacking up giant, over-the-top comparisons to explain how special she is.
Arcangel travels across world geography just to describe her looks. He calls her a monument in Greece, compares her classic vibe to a Venice canal ("Clásica, como canal de Venecia"), and claims she is as rare as the northern lights over the Norwegian coast ("En la costa de Noruega, una aurora boreal"). In Latin American slang, calling someone "dura" means they are extraordinarily attractive or impressive, so he takes the pun literally by saying she is "Dura como la gran muralla China" ("Tough/stunning like the Great Wall of China"). He mixes these colossal landmarks with cozy comforts like morning hot chocolate ("Como el chocolate caliente en el desayuno"), treating this mysterious dream girl like the ultimate luxury he needs all to himself.