Flaca literally translates to "skinny girl". While calling someone skinny in English might be sensitive, in many Latin American countries like Colombia, it's a very common and affectionate nickname for a partner or close friend.
It's a term of endearment similar to 'honey' or 'babe'. In the song, Kapo sings, "Ay, mi flaca hermosa" (Oh, my beautiful skinny girl), using this popular term to express his deep affection and admiration.
Have you ever tried embarrassingly hard to get someone to notice you? That is the exact confession kicking off "UWAIE". The singer comes clean right away with "me he inventado cosas pa' llamar la atención de ti" ("I made things up to get your attention"). It is sweet, clumsy, and completely honest. From there, the track turns into a pure celebration of being head-over-heels. She calls her crush "flaca" (literally "skinny girl," used here as an affectionate pet name), repeating that loving her is not just fun, it is a biological requirement: "Amarte es mi necesidad" ("Loving you is my necessity").
Beyond the big declarations, the lyrics zoom in on goofy, sensory details that make a crush feel real. She wants to dance to an afrobeat rhythm ("bailar contigo un afrito"), shower her with quick kisses ("darte un par de besitos"), and points out how amazing her partner smells while moving together ("Huele rico al bailar"). Best of all, she wants to wash away her partner's doubts with "Quitarte esa inseguridad" ("Taking away that insecurity of yours"). It turns out praying to heaven paid off—"Y si supiera todo lo que yo le recé a Dios / Pa' que tú estuviera' aquí" ("If you only knew how much I prayed to God for you to be here")—and now she is just basking in the prize.