Peda is a very common slang term in Mexico for a drunken party, or more generally, the state of being drunk. The song's title, 'Cosas De La Peda', translates to 'Things That Happen When You're Drunk'.
In the song, the artists sing about the regrettable (and relatable) things people do under the influence, like texting an ex. Learning slang like peda is a great way to understand informal conversations and the real-life culture behind the language.
This song is all about that post-breakup situation where you're definitely not over your ex. Prince Royce gets a 'Happy Birthday' text from his ex after months of silence, and instead of pretending he's doing great, he gets brutally honest. He admits he could lie and say he's moved on, "que en la disco con otra me enredé" [that I got with someone else at the club], but what he really wants to say is "Te extraño, pero borracho no me duele tanto" [I miss you, but when I'm drunk it doesn't hurt as much]. He's drinking from the moment he wakes up just to stop thinking about her.
Then the song flips the perspective. He knows she's doing the exact same thing! He pictures her out partying, "bailando las de Selena" [dancing to Selena songs], and pretending she's fine. But he knows she also misses him and sends him confusing drunk texts that switch from calling him a jerk to saying "Te extraño, mi amor" [I miss you, my love]. The whole song is this messy, relatable cycle of two people who can't let go, using parties and drinking to cope, and knowing that just one text could bring them right back together.