Golondrina translates to "swallow," referring to the migratory bird. It's a beautiful and somewhat poetic word that isn't as common in everyday Spanish as other bird names.
In the song, the lyrics describe the protagonist finding someone "Volando detrás de una golondrina" (Flying behind a swallow). This imagery evokes a sense of freedom, longing, and perhaps a chase after something elusive or fleeting, making it a memorable and evocative choice.
Estopa's "Sola" tells a story of regret and missed opportunities in a relationship. The singer describes a woman who completely captivated him, stopping time in the room and getting tangled in his hair. She was a constant presence, his "paraguas en el chaparrón" [umbrella in the downpour] and his "bandera cuando no era yo" [flag when I wasn't myself].
However, he repeatedly finds her "Sola" [Alone] – drinking the sea in a bar, flying after a swallow, or like a mermaid on the sand. He wonders if it's his fault she doesn't want to be by his side anymore, realizing too late that she felt alone even when she was with him. The line "no la supe entender / Que cuando deje de estar pa' mà / Ella, ella estaba sola" [I didn't understand her / That when she stopped being for me / She, she was alone] really hits hard, showing his painful realization that he failed to see her loneliness until she was gone.