Necio translates to "stubborn" or "foolish." It's a less common word than terco for stubborn, making it more distinctive and interesting.
In the song, the singer directly addresses his heart, pleading, "Aguanta corazón, no seas tan necio" (Hold on, heart, don't be so stubborn/foolish). He's personifying his heart, telling it not to give in to sorrow and pain, highlighting the internal struggle he's experiencing.
This song is a direct plea to a suffering heart, telling it to be strong and not give in to sadness. El Fantasma sings directly to his heart, saying, "Corazón, no es hora de llorar" [Heart, it's not time to cry] and "Aguanta tu penar" [Endure your sorrow]. He's really feeling his heart's pain as his own, admitting, "Tu dolor también es mi dolor" [Your pain is also my pain].
It's a really relatable feeling when you're trying to be tough, but your emotions are just overwhelming you. He even asks, "Corazón, por qué te empeñas en hacerme sufrir" [Heart, why do you insist on making me suffer?], which is such a blunt and honest question. It's like he's having a conversation with his own feelings, urging them to "Mantente en calma" [Stay calm] even though it's clearly a struggle.