Renglón translates to "line" but specifically refers to a line of written text. It's a more formal and less common word than línea.
In the song, the singer recalls writing a letter and crying "en cada renglón" (on every line). This word choice emphasizes the emotional depth and the act of writing, making it more evocative than simply saying línea.
Have you ever looked back at an old letter you wrote during a tough breakup and just laughed at how dramatic you were? That's exactly what Río Roma and Ha-Ash are singing about in "La Última Carta" [The Last Letter].
The song is all about revisiting a letter written in the depths of heartbreak. The singer remembers crying while writing lines like "mi amor, no te vayas nunca de mi lado" [my love, don't ever leave my side] and "me iba a morir sin besos de tus labios" [I was going to die without kisses from your lips]. But now, a year later, they can't help but laugh at those "tonterías que escribe uno enamorado" [silly things one writes when in love]. The best part? They didn't die! In fact, losing that person was "lo mejor que me ha pasado" [the best thing that has happened to me] because it made them stronger and led to a new, true love. It's a fun, honest look at how much perspective can change over time!