Peda is a very common slang word in Mexico for a party, specifically one with a lot of drinking. It can also refer to the state of being very drunk.
In the song, Becky G tortures herself by imagining her ex "en la peda" (at the party), with another girl. Using this colloquial term instead of the more standard word 'fiesta' makes the lyrics feel more raw and authentic, giving you a taste of real, modern Spanish.
This song is a perfect duet for when you're going through a breakup but can't quite let go. Becky G starts things off by staying home alone while her friends go out. She's stuck in her room, listening to their old playlist and looking at photos, feeling totally conflicted: "No sé si decir te amo / Que te odio, que te quiero ver" (I don't know if I should say I love you / That I hate you, that I want to see you). The worst part for her is imagining her ex at a party, doing all the things they used to do, but with someone new. She just wants him to call and say he misses her, even if it's not true: "Aunque sea mentira" (Even if it's a lie).
Then Ivan Cornejo comes in with his side of the story, and it turns out he's feeling the exact same way! He remembers her saying she couldn't live without him, "Sin mí te morirías / Pero sigues viva" (Without me you would die / But you're still alive), which is such a blunt and sad line. He admits his own sun doesn't shine without her, "mi sol ya no brilla", but he still hopes she's happy. They both just want to hear that the other person is miserable without them, making this a powerful song about two people who are apart but sharing the same heartbreak.