Compadezcas comes from the verb compadecer, which means 'to pity' or 'to feel compassion for'. It's a deeply emotional word that you don't hear every day, carrying more weight than simply feeling sorry for someone.
In the song, the singer pleads, "Te estoy hablando con el corazón para que te compadezcas" (I'm speaking to you from the heart so that you'll take pity). He's so heartbroken that he's moved beyond asking for love and is now just begging for her compassion, making this a powerful and dramatic word that captures the raw desperation of the lyrics.
This song is a full-on, desperate plea from someone who is not handling a breakup well. Carin Leon and Maluma sing from the perspective of a guy who feels completely alone, even when he's surrounded by people: "Tengo un millón de personas y me siento solitario" (I have a million people and I feel lonely). He's a mess. He's not sleeping, he's looking at old photos, and he's talking to himself about the relationship.
The whole point of the song is in the title. He keeps repeating, "Si tú me vieras" (If you saw me), convinced that if his ex could only see how much he's suffering, she'd take him back. He's drinking to forget her, but it's not working: "Yo sé que superarte debo / Pero tu nombre vuelve cuando bebo" (I know I should get over you / But your name comes back when I drink). It's a dramatic, heartfelt attempt to win her back by showing her just how miserable he is without her.