Asfixia'o is a colloquial, shortened form of "asfixiado," meaning "suffocated." It's an intense and somewhat dramatic word that isn't commonly found in every song.
In this song, Omar Courtz uses it to express being completely overwhelmed and consumed by his feelings for someone, almost to the point of being unable to breathe. He sings, "A-a-a-asfixia'o contigo, me hiciste un vudú" (Suffocated with you, you put a voodoo spell on me), highlighting the powerful, almost magical hold this person has over him.
Omar Courtz and Karbeats open this one with a fake news bulletin about the end of the world, then flip straight into a horny, half-joking reggaetón built on that bit: since the apocalypse is coming anyway, why not spend it with her. "Mami, están diciendo que el mundo se va a acabar" [Babe, they're saying the world's about to end] is the hook, and "Me tienes loquito, ¿cuándo es que te vas a trepar encima?" [You've got me crazy, when are you climbing on top?] sets the tone for everything after it. It's blunt on purpose, mixing phone-era slang ("Cada vez que la veo 'esnúa, me quedo loading" [Every time I see her naked, I just start buffering]) with a whole verse about staying in and watching Netflix instead of going out.
By the second half the joke gets a little sincere. "¿Dónde vamos a acabar?" [Where are we going to end up?] reads less like a punchline and more like an actual question about whether this is going anywhere, and he answers it himself: "Nunca vamos a acabar, el cuento de nunca acabar" [We're never going to end, the never-ending story]. The end of the world turns out to be the excuse he needed to admit he's not trying to end this.