Manzanita literally translates to "little apple", but its meaning here is much sweeter. It's a charming piece of Mexican slang used as an affectionate compliment.
In the song, the phrase "Pura manzanita" describes someone as being wholesome, lovely, and top-quality, just like a perfect little apple. Ivan Cornejo uses this unique term of endearment to admire the girl's smile, calling it a delicia (delight) and emphasizing her sweet nature despite the heartache she's endured.
Ivan Cornejo tells a moody heartbreak story in "Está Dañada", watching a girl put her walls up after getting hurt. He sets the scene right away: "Está dañada del amor" ("She is damaged by love") and "Llueve cada día en su mundo" ("It rains every day in her world"). Her faith in romance is completely gone, to the point where she goes numb and feels nothing at all. But she finds an escape on nights out. When she dances to reggaeton with her friends, her mood flips and her beautiful smile returns—he even calls her cute face "pura manzanita" ("pure little apple"), an affectionate Mexican phrase used to describe smooth, rosy, apple-like cheeks.
Then the song turns personal. Ivan pulls from his own memories of dancing and drunkenly kissing her ("Que bailamos, nos besamos bien borrachos"). Even though things fell apart, he knows her nights out only mask the pain. He leaves her with one blunt wish: "Y cuando estés llorando / Encerrada en tu cuarto / Espero que tú cantes mi canción" ("And when you are crying, locked in your room, I hope that you sing my song"). He wants his voice to be her company when the party ends and the sadness catches back up.