Desmadre is a versatile and very common slang term in Spanish, especially in Mexico, that literally means "chaos" or "mess". However, in the context of this song, it refers to a lifestyle of wild partying, irresponsibility, and general debauchery.
Carín León sings, "Por ti me retiro del desmadre" (For you, I'm retiring from the chaos/wild partying). He's promising to give up his wild ways and settle down for the person he loves, making this word central to the song's theme of transformation and commitment.
Carín León opens the song reading out his own bad reviews. Someone has already briefed this girl on him: "Que soy una mierda, que soy un infiel" (That I'm a piece of sh*t, that I'm unfaithful), "Y que vivo en la peda" (And that I live in the party). He does not argue with a word of it. His entire defence is five words long, "Pero eso era con ellas" (But that was with them), followed by the hook that carries the rest of the song, "Porque ya encontré a la buena" (Because I already found the good one). It is the oldest promise in regional Mexican music and he delivers it completely straight.
What makes it fun is how small and concrete the promises get. The proof he offers is a new phone number, "Cambié el número de una / Pa' que no me hable ninguna" (I changed my number right away / So that none of them can call me). And the reason he will be too busy for the old crowd is not a noble one: "Que yo andaré ocupado bajándote el panty" (Because I'll be busy pulling down your panties). The bridge holds the best joke on the record, where he works out what his behaviour has cost him and decides he can live with the bill: "A lo mejor, baby, por mis cagadas / Me niegan la entrada pa'l cielo" (Maybe, babe, because of my screw-ups they'll deny me entry to heaven), "Pero la neta que ni me hace falta" (But honestly, I don't even need it).