Mentiras translates directly to "lies." While it might seem like a common word, its prominence as the song's title and its repeated use in the chorus make it incredibly impactful and central to the song's theme.
In the context of Daniela Romo's song, "Mentiras" encapsulates the entire emotional struggle of the narrator. She's grappling with a partner who constantly deceives her, making it impossible to trust or continue the relationship. The word perfectly conveys the heartbreak and frustration of being fed falsehoods.