Rienda literally translates to "rein," like the straps used to guide a horse. It's an interesting word because it's not commonly found in everyday Spanish pop songs.
In the lyrics, "Tengo suelta la rienda, no tengo freno," the phrase "suelta la rienda" means to let go of the reins, implying a lack of control or inhibition. The singer is expressing a carefree and impulsive attitude, ready to give in to desire without holding back.
"Mal De La Cabeza" is a flirty, high-energy track about being completely infatuated with someone to the point of acting a little crazy. Mau y Ricky and Becky G trade playful lines about staying up late, chasing each other around, and losing their heads over an irresistible attraction.
The chorus repeats the title phrase, "mal de la cabeza" ("sick in the head" or "crazy in the head"), as a lighthearted way of saying love and desire can make you act a little unhinged. The verses lean into teasing, suggestive back-and-forth between the two voices, painting a fun night out where neither one can resist the other.
It's a song about that dizzy, can't-think-straight feeling of being drawn to someone — embracing the chaos instead of fighting it.