Márchate is the imperative form of the verb marcharse, meaning "to leave" or "to go away". While "to leave" might seem common, the use of the imperative here is particularly strong and emotionally charged.
In the context of the song, the singer is telling his unfaithful or indifferent lover to "just leave" because he can no longer bear her pretense. It's a definitive and somewhat dramatic command, making it a memorable and impactful word that drives the song's central theme of a relationship ending.
Darío Gómez brings us a song about a man who's had enough of being treated poorly. He's asking his partner directly, "Dime por Dios, mujer, ¿qué es lo que pasa? ¿Por qué me tratas tan indiferente?" [Tell me for God's sake, woman, what's happening? Why do you treat me so indifferently?].
He's not going to beg for love. He makes it clear that if her affection has been fake all along, she should just leave: "Sin has fingido todo el tiempo tu cariño, Mejor márchate, mujer, no quiero verte" [If you've faked your affection all this time, it's better you leave, woman, I don't want to see you]. He wants the truth, asking her to tell him "mirándome a los ojos" [looking me in the eyes] if she doesn't want to be with him anymore. It's a blunt, no-nonsense approach to a relationship that's clearly on its last legs.