Desapercibida means "unnoticed" or "unperceived." It's a less common, more evocative word than simply saying something isn't seen.
In "Un hombre normal," the singer uses this word to express his intention to hide his sadness. He plans to smile "para que pase desapercibida mi tristeza" (so that my sadness goes unnoticed), highlighting his effort to conceal his true feelings from his former love.
Have you ever tried to act totally cool and unaffected after a breakup, even though you're completely heartbroken? That's exactly what Espinoza Paz is singing about in "Un Hombre Normal" [A Normal Man]. He's telling his heart, "A través de esta canción, quiero decirte, cómo me siento" [Through this song, I want to tell you how I feel]. He knows the person he's singing to has lost interest, admitting, "Nunca fui tu prioridad, ni tu centro de atención" [I was never your priority, nor your center of attention]. Ouch! He's really trying to come to terms with the fact that he's no longer in their heart.
But here's the twist: he's going to pretend to be "un hombre normal / Que puede estar sin ti, que no se siente mal" [a normal man / Who can be without you, who doesn't feel bad]. He even promises, "por primera vez, prometo no llorar" [for the first time, I promise not to cry]. It’s a classic move, right? Putting on a brave face and smiling so his sadness goes unnoticed. You can almost feel him trying to convince himself as much as the other person. It’s a really relatable song about trying to handle a tough goodbye with a little bit of dignity, even when your heart is breaking.