Cejas literally means "eyebrows". It's a surprisingly specific and uncommon word to find in a love song, which is what makes it so memorable and interesting.
In "Atención," the singer is so captivated by a person he barely knows that he's listing every little detail that might have attracted him. By singing, "O algo de tus cejas" (Or something about your eyebrows), he shows the depth of his infatuation, where even a minor detail becomes a powerful point of attraction.
Ever catch someone's eye and instantly imagine your entire future together? That's exactly what Ivan Cornejo is singing about in "Atención." He gets one look at a stranger and his world flips. He admits, "Vi el futuro nuestro, y de ti no sé nada" (I saw our future, and I know nothing about you). Now he can't stop thinking about them, wondering if they even know he exists. He's so fixated that he begs, "Mírame y dime dónde puedo encontrarte una vez más / Pa' terminar lo que empezó" (Look at me and tell me where I can find you one more time / To finish what started). It's a bold line, considering nothing has actually started yet!
This song is all about wanting to be seen by the person you're crushing on. Cornejo feels completely invisible, pleading, "Quiero tu atención y no tu espalda" (I want your attention and not your back). He runs through a list of everything he found attractive, from their lips and hands to, hilariously, "algo de tus cejas" (something about your eyebrows). He's totally obsessed. The song ends by repeating the main point: he feels all of this for someone he doesn't even know, singing, "Y eso que no conozco quién eres" (And that's without even knowing who you are).