Rompecabezas literally translates to "head-breaker", but it's the common Spanish word for "puzzle". It's an engaging and somewhat unexpected word to find in a romantic song.
In the lyrics, the singer contrasts the simplicity of love with the complexity of a "rompecabezas" (puzzle), implying that love shouldn't be as complicated as it's being made out to be. This metaphorical use makes the word stand out and adds a layer of intrigue to the song's message.
Maka's "Rompecabezas" is a passionate plea from someone deeply in love, trying to understand why their affection isn't being reciprocated. The singer is giving their all, offering their life and the "nobleza que tiene mi corazón" [nobility that my heart has], but feels it's being rejected.
He sees love as simple, much easier "Que todo este rompecabezas" [Than all this puzzle], and urges his beloved to shed her fear and kiss him. He wants her to take him, to not let him go, because he believes he's the one who truly loves her. The lyrics are full of longing, with lines like "Eres por quien de noche sueño y por quien vivo de día" [You are who I dream of at night and who I live for by day], and a fear of losing her, wondering how she could doubt his intense feelings.