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.
This song is all about a messy, on-again, off-again relationship. Enrique Iglesias describes a love that's constantly "sube y baja" (up and down), but somehow, it just works. One minute, one of them says, "se acabó" (it's over), but then a little voice inside says "no," and they stay together. It's that kind of connection that feels impossible to quit, even when it's difficult.
But here's the twist. He's not begging his partner to stay. Instead, he's making a super confident, almost cocky, prediction. He says, "Si decides dejarme / No te voy a suplicar" (If you decide to leave me / I'm not going to beg). He's basically telling her to go ahead and try to leave, because he knows she won't be able to forget him. He warns her, "Aunque corras, te escondas, no puedes escapar" (Even if you run, you hide, you can't escape). It's a bold claim that their love, good or bad, is too real to ever truly leave behind.