Me arrepiento means "I regret it." It's a powerful and emotional phrase that comes from the reflexive verb arrepentirse, which is a great grammar concept to learn.
In the song, Alvaro Soler sings, "tú no sabes cuánto me arrepiento" (you don't know how much I regret it), expressing deep remorse for being 'blind' and focusing on material wealth instead of his love. This word is the emotional heart of the song, capturing his moment of profound realization.
Alvaro Soler has had a huge wake-up call in this song. He admits he was chasing all the wrong things, staying on the surface—"Me quedé en la superficie" [I stayed on the surface]—and always wanting more and more. He realizes he was so focused on material possessions that he was completely blind to what really mattered, saying "Estaba ciego / Y ya no vi nada más" [I was blind / And I saw nothing else]. It's a classic case of not knowing what you've got 'til it's gone, as he sings, "hasta que tú no lo pierdes todo / No te das cuenta de nada" [until you lose everything / you don't realize anything].
So what's the point of all that wealth? According to him, nothing! He lists off an incredible collection of luxury goods: "Una casa en una isla" [A house on an island], "Diamantes y cien coches" [Diamonds and a hundred cars], and even "Aviones, mansiones" [Airplanes, mansions]. But he makes it very clear that none of it means a thing without the person he's singing to. Every list of riches ends with the same simple, powerful line: "Si no te tengo a ti" [If I don't have you].