Pedo literally means "fart" in Spanish, but in Mexican slang, it's commonly used to mean "problem," "issue," or "business" (as in, "it's my business"). It's a very colloquial and versatile word.
In this song, Christian Nodal sings, "De sus besos que les valen, muy mi pedo" which translates to "About her kisses that you don't care about, that's very much my problem/business." He uses it to assert that his feelings and memories are his own concern and no one else's, adding a defiant and personal touch to the lyrics.
Christian Nodal is laying down the law in "Ni Se Metan"! This song is all about setting boundaries when you're going through a tough breakup, and he's not holding back. He's basically telling everyone to mind their own business, asking, "¿Quién les preguntó? ¿Quién pidió su opinión?" [Who asked you? Who asked for your opinion?]. He makes it clear that his pain is his own, saying, "Era el amor de mi vida, no el de ustedes" [It was the love of my life, not yours].
He's still deep in his feelings, remembering "sus besos que les valen, muy mi pedo" [her kisses that mean nothing to you, that's my problem]. And if he has a few too many drinks and gets the urge to go beg at her door, he warns everyone, "Ni se metan" [Don't even get involved]. It's a blunt and honest look at heartbreak, and Nodal isn't afraid to tell everyone to back off and let him deal with it his way.