Agrieta comes from the verb agrietar, meaning to crack or split. It's a vivid and somewhat dramatic word that paints a strong picture of damage or breaking.
In the song, the artist uses it metaphorically to describe the pain of a broken heart: "lo que siente un corazón cuando se agrieta" (what a heart feels when it cracks). This imagery makes the emotional impact of the lyrics much more profound and memorable.
Have you ever felt like you gave someone everything, only for them to leave and you're left wondering what went wrong? Santa RM's "Quizás" [Perhaps] dives right into that feeling. He sings about being forgotten by someone he can't forget, saying, "cuando has estado quizás no me recuerdes pero yo no te olvidado" [when you've been perhaps you don't remember me but I haven't forgotten you].
He blames himself for giving his all, calling himself "el idiota que te creía el idiota que te quería" [the idiot who believed you, the idiot who loved you], realizing now that everything she said was a lie. Even though he knows it's probably not true, he still hopes she'll come back and say she loves only him. It's a really raw look at heartbreak, with lines like "quien me saque el corazón que alguien me ayude porque intenté olvidarte el oscuro pero no pude" [whoever takes my heart out, someone help me because I tried to forget you in the dark but I couldn't]. He even says he'd give his soul for one more kiss, showing just how much he's still longing for her.