Disimular means "to hide" or "to conceal," but it's specifically used for hiding your true feelings or intentions. It's a powerful verb that captures the pressure to pretend everything is okay when it's not.
In the song, Aitana asks the heartbreaking question, "¿Por qué disimular que todo está bien si no está bien?" (Why hide that everything is okay if it's not okay?). This word is at the core of the song's message about the struggle of maintaining a perfect image.
This song is all about the pressure to be perfect and finally deciding you've had enough! It starts by describing this girl who everyone thinks is a "diez de diez" [a ten out of ten]. From the outside, she does everything right and never steps out of line. But on the inside? The song is super direct about it: "ella está teniendo unos días de mierda" [she's having some shitty days] and feels so anxious she can't even go outside. She's crying, and nobody even notices.
So, she gets tired of it! She's done feeling like she "pertenece a todos / Cuando es solo de ella" [belongs to everyone / When she only belongs to herself]. The song asks why she has to put on a happy face when she's had a bad week, or why she has to dress perfectly when she feels miserable inside. She decides to break free, and people immediately start gossiping about how she's changed or if she's had surgery. But she knows what she wants and doesn't care anymore. The chorus is her big declaration: "Ella no quiere ser la chica perfecta" [She doesn't want to be the perfect girl].