Prendiendo comes from prender, meaning “to light,” “ignite,” or “turn on.” In the song, “se nos fue prendiendo el fuego” uses it metaphorically to describe love gradually catching fire between two people, making this a vivid and memorable expression.
Tito Double P, the Mexican singer behind a lot of today's corridos, slows things down here for a love story that never really ended. Two people are living far apart now, sent off by life "Por diferentes coordenadas" [in different directions, literally "coordinates"], but they still talk "Como si nada cambió jamás" [as if nothing had ever changed]. He calls it "una conexión cabrona" [a damn strong connection]. Cabrona is Mexican slang that can be an insult, but here it works as a rough compliment, like saying something is crazy good. It is the bluntest word in the song and also the most honest one.
The best part is how it started. He swears he wasn't looking for anything: "Yo no buscaba amores nuevos" [I wasn't looking for new love], he just went out "tranquilo" [relaxed]. Then "el destino hizo su juego" [fate played its hand], and between laughs and looks "Se nos fue prendiendo el fuego" [the fire slowly caught]. What he remembers most is being himself with her, "Sin mentiras ni disfraces" [without lies or disguises]. And the setting is real: Ensenada is a port city on the Pacific coast of Baja California, so when he asks "¿Cómo olvidar en Ensenada?" [How could I forget, in Ensenada?], picture nights by the sea "Mientras la luna nos miraba" [while the moon watched us]. The moon watching them is a bit much, sure, but it is a corrido, and he means every word.