Allá tú is a fantastic idiomatic expression that literally means "over there, you". In conversation, however, it's a sassy way to say "That's on you," "It's your problem," or "Suit yourself."
In the song, Enrique sings "Si decides dejarme... Allá tú si más tarde..." (If you decide to leave me... that's on you if later...). He's defiantly telling his lover that the decision and its future consequences are theirs alone. It's a great colloquial phrase that will make you sound more like a native speaker.
Spanish pop star Enrique Iglesias is laying all his cards on the table in "Escapar." Right from the start, he admits the relationship is a chaotic rollercoaster—"Sube y baja pero un día al fin / Sin querer nos va bien" ("It goes up and down, but in the end, one day / Without meaning to, we do well"). Even when somebody calls it quits with "se acabó" ("it's over"), another voice steps in, refuses to budge, and stays put.
Then comes the ultimatum, delivered with pure confidence. Enrique flat-out refuses to beg if you walk away ("No te voy a suplicar" / "I am not going to beg you"). Instead, he drops a blunt Spanish idiom: "Allá tú", which roughly translates to "that's on you" or "suit yourself." He warns that trying to move on will not work anyway: "Aunque corras, te escondas, no puedes escapar" ("Even if you run, if you hide, you cannot escape"). In his eyes, what they have tastes sweet and bitter all at once, but it is too real to leave behind.