Me arrepiento means "I regret". It comes from the important reflexive verb arrepentirse, which is used to express a deep sense of remorse for something you've done wrong.
This phrase is the emotional core of the song. The singer repeats "yo me arrepiento" in the chorus, emphasizing their regret over past actions and their desperate wish to make things right with the person they still love.
This is a classic pop duet about two people who have broken up and are both completely miserable without each other. Abraham Mateo and Ana Mena trade verses, singing about how much they regret what happened and how desperately they want to get back together. The feeling is mutual, and they both admit, "ahora que no estás aquí / Que me di cuenta de lo que perdí" (now that you're not here / I realized what I lost).
They paint a pretty dramatic picture of their heartbreak. The house feels like hell where "Queman los recuerdos" (memories burn), and they feel like they're drowning in sadness. The core of the song is a direct apology and a plea for a second chance. They both sing the main chorus line, "Quiero decirte que lo siento, que te echo de menos" (I want to tell you I'm sorry, that I miss you), making it clear they still love each other and haven't given up hope.