Allá tú is a common colloquial phrase that literally means "there you". It's used to put the responsibility for a decision or its consequences onto another person, similar to saying "That's on you" or "Suit yourself".
In the song, the singer says "Allá tú si te alejas de mí" (That's on you if you distance yourself from me). It's a powerful expression of resignation and defiance, telling his love that the choice to leave is theirs to make—and their burden to carry.
This song is all about a love that feels unbreakable, even when it's falling apart. The singers recall their past together, insisting, "De las noches que pasamos / Nadie podrá separarnos" (Of the nights we spent / Nobody can separate us). But something has clearly gone wrong. The singer feels completely left behind, listening to a song called "Perro Abandonado" (Abandoned Dog) and saying, "así me dejaste" (that's how you left me). It's a really direct and sad image.
Despite the heartbreak, there's a lot of fight left in them. They have to deal with their parents, who think their ex isn't good enough for them: "mis padres dicen que no te quieren / Porque dicen que no me mereces" (my parents say they don't like you / Because they say you don't deserve me). But the singer doesn't care and vows to keep trying. The song ends with a mix of longing for the past and a bold warning for the future: "No volverás a encontrar lo que teníamos" (You won't find what we had again).