Destá is a colloquial and somewhat regional interjection in Portuguese, often used to express a sense of triumph, vindication, or to say "There you go!" or "Take that!"
In the context of the song, the singer uses Destá repeatedly to convey a feeling of eventual victory and payback. He's telling an ex-lover, "There you go, you'll come knocking on my door," implying that she will regret leaving him and he will have the last laugh. It's a very expressive and engaging word that captures the song's theme of poetic justice.
This is a forró revenge song dressed up as a promise. The singer isn't begging his ex to come back, he's telling her she will, on her own, eventually: "Destá, eu hei de ver você bater em minha porta" (Just wait, I'll see you knocking on my door). "Destá" is the whole attitude packed into one word, a shrug that means something like just wait and see.
He isn't bitter about being left for someone else, he's patient about it, repeating "quem rir por último rir melhor" (whoever laughs last laughs best) like he's already counting the days. Even his dream gives him the ending he wants: her crying, second-guessing, "reagiu sem perceber" (she reacted without realizing it). By the time the accordion kicks back in for "Eita forrozão!" (what a forró!), the joke and the song land the same way.