Romperlo translates to 'to break it.' The verb is romper (to break), and lo is a pronoun meaning 'it' that is attached to the end of the verb.
In this song, the unspoken 'it' is the singer's love or heart. He sings the devastating line, "¿Qué sentido hay en regalarte mi amor para romperlo así de fácil?" (What's the point of gifting you my love, only to break it so easily?). This one word powerfully captures the pain of having a precious gift be carelessly shattered.
Alvaro Soler is not holding back in this song! It's a classic heartbreak anthem, but with a very direct and personal feel. He tells the story of a sudden breakup that left him completely confused. One minute, he thought they'd be together forever, saying, "Nunca llegué a pensar / Que no seríamos tú y yo" (I never thought / it wouldn't be you and me). The next, his partner is gone, leaving him wondering what he did wrong.
He dedicates the entire song to the person who hurt him, singing "Te dedico esta canción" (I dedicate this song to you). He's left with a heart that "Ya no habla de amor" (no longer speaks of love) because of all the pain. It's a simple, powerful message about being left behind and trying to make sense of a love that was broken "así de fácil" (so easily).