Farsa means 'farce' or 'sham'. It's a bold, theatrical word that you don't hear in every pop song.
In this context, the singer tells the other woman that her belief she owns the man is just "una farsa". The word carries a dramatic, almost scandalous tone, perfectly setting up the song's fiery love triangle. Learning it gives you a powerful way to call out something that's completely fake.
This song is one of the most direct pop songs you'll ever hear! María José isn't playing the part of a heartbroken secret lover. Instead, she's singing directly to her lover's main partner and telling her exactly how things are. She lays it all out, saying the other woman's belief that he belongs only to her is a lie: "Crees que es tuyo solamente / Pero es una farsa" (You think he's only yours / But it's a farce). The singer claims the man is tired of his partner, who has become just "rutina" (routine).
In contrast, the singer has all the good parts. She boasts, "yo tengo sus noches y su pasión / Sus fantasías y su obsesión" (I have his nights and his passion / His fantasies and his obsession). She even confirms the other woman's suspicions aren't crazy, declaring proudly, "hoy entérate / Yo soy la otra" (find out today / I am the other woman). The whole song is a bold statement that she prefers this role, ending with the powerful line, "Prefiero ser su amante" (I prefer to be his lover).