Montón literally means "a heap" or "a pile," but in everyday Spanish, it's very commonly used to mean "a lot" or "a great deal."
In this song, the singer uses it to express the immense gratitude his heart will feel if his former love doesn't reappear. He sings, "Mi corazón te va a agradecer Un MONTÓN," meaning "My heart is going to thank you A LOT." It's an interesting and emphatic way to convey a strong emotion.
Novio's "Montón (Acústico)" is a really relatable song about that confusing, painful moment when you realize an ex has moved on, but you're still stuck in the past. He sings, "Te he buscado / Pero ni te has enterado" [I've looked for you / But you haven't even noticed], which perfectly captures that feeling of trying to reconnect only to find they're completely oblivious.
It's clear he's been hurt, especially when he mentions, "Ya me han contado / Te vieron con él de la mano / Mi corazón en mil pedazos" [They've already told me / They saw you with him hand in hand / My heart in a thousand pieces]. But even with a broken heart, there's a glimmer of hope for healing. He repeats, "Si no vuelves a aparecer / Mi corazón te va a agradecer / Un MONTÓN" [If you don't reappear / My heart will thank you / A LOT], showing that while it's hard, he knows moving on is the best thing for him in the long run. It's a bittersweet journey of remembering the good times ("no hay día nena / Que no piense en las maneras / En que me entregabas tu amor" [there's not a day, baby / That I don't think of the ways / You gave me your love]) while also trying to find peace.