Pena ajena is a fascinating Spanish phrase that doesn't have a perfect one-word English equivalent. It describes the feeling of secondhand embarrassment or 'cringe' you feel on behalf of someone else when they do something awkward or foolish.
In this song, the singer delivers a powerful insult, telling his ex, "vete porque causas pena ajena" (leave because you cause me secondhand embarrassment). He's saying her desperate attempts to get him back are not just sad, but truly cringeworthy to witness.
This song is the opposite of a sad breakup ballad. It's a powerful, direct, and even a little bit cruel message to an ex. The singer is completely finished with the relationship and makes it crystal clear.
He tells his ex he's not coming back, no matter what she does: "Dime cómo quieres que te explique / Que por más que llores y supliques / Yo ya no vuelvo contigo" (Tell me how you want me to explain / That no matter how much you cry and beg / I'm not getting back with you). He's so done that he even offers her money just to go away for good: "Cuánto quieres que te pague / Solo quiero que te largues / Te lo doy en efectivo" (How much do you want me to pay you / I just want you to get lost / I'll give it to you in cash). It's a very blunt way to say "it's over."