Tapo comes from the verb tapar, meaning "to cover" or "to block out". While the word itself is common, its use here is part of the famous Spanish idiom: "tapar el sol con un dedo" (to cover the sun with a finger).
This phrase describes a futile attempt to hide something obvious and immense. In the song, the singer uses this powerful metaphor to express how he's trying to deny that he misses his ex, an impossible task, much like blocking out the entire sun with just one finger.
Venezuelan pop singer Lasso delivers an honest, catchy look at a date that goes perfectly right on paper, yet completely wrong in the heart. He is sitting in the same restaurant he used to visit with his ex, repeating to himself, "Estoy bien" ("I am fine"). On the surface, this new partner is ideal. She laughs at his jokes, gets along with his friends, and they never argue—which he admits is "lo que siempre he querido" ("what I have always wanted"). They look good holding hands, her lips are red, and her eyes are green. Yet the second he looks into her face, the illusion shatters: "Veo que no son tus ojos marrones" ("I see that they are not your brown eyes").
No matter how hard he tries to move on, the comparison is instant. He describes his world losing its brightness: "El cielo azul parece gris" ("The blue sky seems grey") and everything turns black and white. Lasso also brings in a classic Spanish idiom when he admits, "Mejor, al sol, lo tapo con un dedo" (literally "Better to cover the sun with a finger"). In Spanish, "tapar el sol con un dedo" means trying to hide or downplay a massive, undeniable problem with a useless, tiny fix. He cannot turn back time, so he pretends he is fine, even though the chorus drives home the real truth: absolutely nothing is the same without those brown eyes.