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.
Have you ever locked eyes with a complete stranger and immediately started imagining your whole life together? That is the exact spiral Ivan Cornejo falls into in "Atención". After sharing a single glance with someone, he realizes he forgot to ask for their name: "Te miré, y no pude preguntar por tu nombre" (I looked at you, and I couldn't ask for your name). That one missed chance drives him crazy, keeping him up 24/7 wondering if the other person even knows he exists. He wants them to tell him where to meet up right away, using the everyday Mexican slang "en caliente" ("Que en caliente yo me voy" [Because I'll go right away / without hesitation]), to finish whatever started between them.
The real hook of the song is the pure frustration of being ignored. Ivan admits he cannot stop fixating on everything from their lips to their eyebrows ("o algo de tus cejas" [or something about your eyebrows]), but in reality, he is staring at the back of their head. He literally pleads, "Quiero tu atención y no tu espalda" (I want your attention and not your back), reminding them with a touch of desperation that he isn't invisible: "No soy fantasma" (I'm not a ghost). It is dramatic, a little obsessive, and completely relatable when you realize he does not even know this person at all ("Y eso que no conozco quién eres" [And that's despite not knowing who you are]).