Desvelo comes from the verb desvelarse, meaning to stay awake or to lose sleep. It's a more evocative word than simply saying "no duermo" (I don't sleep).
In the song, the singer uses it to express how much he misses his former lover, stating, "Yo también te extraño también me desvelo" (I also miss you, I also stay awake). It conveys a deep emotional impact, suggesting that his longing is so intense it keeps him from sleeping.
This song is a fun, slightly sassy look back at a past relationship! The singer misses their ex, admitting, "Yo también te extraño, también me desvelo viendo nuestras fotos y videos" [I miss you too, I also stay awake watching our photos and videos]. They can't stop thinking about their ex's lips, imagining kissing them again.
But it's not all longing! The singer playfully challenges their ex, asking, "De los besos que te di, amor, ¿a cuál de todos echas más de menos?" [Of the kisses I gave you, love, which one do you miss the most?] They wonder if it's the "tiernos con los que llevaban veneno" [tender ones that carried poison] or the "inocentes, esos que no tenían frenos" [innocent ones, those that had no brakes]. The singer even teases, "Si tú solita fuiste quien me dio luz verde, ahora resulta que hasta la memoria pierdes" [If you yourself gave me the green light, now it turns out you're even losing your memory]. It's clear they believe their ex remembers them, especially at night, and still longs for those kisses!