Peló comes from the verb pelarse, which is a colloquial and somewhat informal way to say "to leave quickly" or "to run off". It implies a sudden or hasty departure, often without much explanation.
In the song, the lyric "Porque tú fuiste la que se peló de mi lado" (Because you were the one who left my side) uses peló to emphasize that the ex-partner abandoned the relationship abruptly. It's a more vivid and less common word than simply saying "se fue" (she left), making it memorable and adding a touch of emotional weight to the narrative of betrayal and moving on.
This song is a total kiss-off anthem! Xavi is telling an ex-girlfriend exactly where to go now that he's successful. He starts by setting the scene: he's put his phone on Do Not Disturb mode, or "DND", because he's getting so much attention. He says, "Antes no querías, ahora quieres verme" (Before you didn't want me, now you want to see me), because he's got a "bolsita llena de billetes" (a little bag full of cash).
He's not holding back at all. He reminds her that she was the one who left him, saying "tú fuiste la que se peló de mi lado" (you were the one who took off from my side). Now that she wants to come back, he's moved on and sends her calls straight to voicemail. He ends the chorus with the very blunt line, "se me pegó la gana de mandarte a la chingada" (I felt like telling you to f*** off). It's a classic story of someone getting over a breakup and finding success, and then having to deal with the person who left them trying to come back.