Raso can mean 'flat', 'smooth', or 'ground level', but in the context of "al raso," it means 'in the open air' or 'out in the open'. It's a less common way to express being outdoors.
In the song, the lyrics "Nos quedamos dormidos al raso mirando el ocaso" paint a romantic picture of falling asleep under the open sky while watching the sunset. This evocative imagery makes raso an interesting and memorable word.
Estopa's "No Digo Ná" drops us right into a romantic scene, with the singer and his love falling asleep under the stars, watching the sunset. He feels time slows down when he's with her, saying, "Que lento va el tiempo siguiendo tus pasos" [How slow time goes following your steps].
He's head over heels, admitting he feels like a clown by her side ("Si estoy a tu lado me siento un payaso") but completely lost without her. He even says he was better off alone, "en el culo de un vaso" [at the bottom of a glass], before she came along like a delayed train. But now, with her sad eyes, he can't breathe without her. He's tired of being the "lucero que apagará la hoguera" [star that will put out the bonfire] and wonders who will free him from his emotional chains. Despite all these intense feelings, his repeated line is, "Yo no digo ná" [I don't say anything], which is pretty funny and relatable if you've ever been speechless in love! He just wants to stop time and live in the moment with her, calling her the princess of a story he never wants to end.