Rastri literally means 'rake' or 'comb'. However, in the context of this reggaeton song, it's used as a slang term with a suggestive double meaning, referring to a sexual act.
Quevedo sings, "Y acabé dándole rastri, je" (And I ended up giving her rastri, heh), implying a sexual encounter. This word is interesting because it's a clever play on words, using an everyday object to hint at something more provocative, making it memorable and unique to the song's style.
"DURO" is a boast written at one specific person: the man whose partner the narrator spent the night with. The claim and the sting arrive in the same breath — "Anoche salí con tu wifey / Y no parecía tu wifey" ("Last night I went out with your wifey, and she didn't act much like your wifey"). What follows is told in small concrete pieces rather than in general terms: low light, a back seat, keeping quiet in the taxi, and then "Nos metimo' en la Ford Transit" ("We got into the Ford Transit") because her parents were home. The chorus is one word repeated past the point of meaning anything but insistence — duro, hard.
The second verse drops the story and speaks to the man directly. "Esa no es tu gata, cabrón / Aunque tú le pusieras el anillo" ("She isn't your girl, man — even if you were the one who put the ring on her") is the argument, and "Que ustedes no chingan desde que se lo di yo" ("You two haven't slept together since I did") is the twist of the knife. The quietest line does the most work: "Se maquilla tarareando mis estribillos" ("She does her makeup humming my choruses") — his songs go with her when she gets ready, which is the part the other man cannot answer.