Cuídala is the imperative form of the verb cuidar (to take care of, to look after), combined with the direct object pronoun la (her). It's a direct and powerful command.
In this song, the singer, who has lost his love, pleads with her new partner to "take care of her" because she is truly special. It's a poignant and memorable word that encapsulates the song's central theme of regret and the importance of cherishing someone unique.
This song is a message from a man to his ex-girlfriend's new partner. Talk about an awkward conversation! He starts by admitting she's probably not talking about him, and he can't blame her. He sees she's moved on and healed. Then he drops a secret detail: they share a matching tattoo. He says, "Ese tatuaje que no enseña y guarda / Es sólo medio corazón / Y la otra mitad la tengo yo en mi espalda" (That tattoo she hides and doesn't show / Is only half a heart / And I have the other half on my back). He accepts that the new guy has taken his place, and that's why he's reaching out.
His whole message boils down to one word: Cuídala (Take care of her). He begs the new boyfriend to treat her right because, as he says, "diosas como ella no existen ya" (goddesses like her don't exist anymore). He calls himself an idiot for losing her and doesn't want someone else to make the same mistake. It's a song full of regret, where the singer warns his replacement not to mess up the way he did, because he only realized he "perdió lo mejor que tenía" (lost the best thing he had) when it was too late.