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.
This song is about a girl who is completely over love. As Ivan Cornejo sings, "Está dañada del amor" (She's damaged by love) and "Ya no cree en el amor" (She doesn't believe in love anymore). It's like her world is permanently sad; the song says, "Llueve cada día en su mundo" (It rains every day in her world). She's lost all hope and happiness when it comes to romance.
But the singer sees something else. He still thinks she's beautiful, admiring her "carita" (little face). He notices that she finds a little bit of joy when she goes out dancing with her friends. This is where the story gets personal. He remembers a specific night with her, singing about the song "Que bailamos, nos besamos bien borrachos" (That we danced to, we kissed each other very drunk). Now, his final wish for her is a little heartbreaking. He hopes that when she's alone and sad, "Encerrada en tu cuarto" (Locked in your room), she'll sing his song.