Resbala comes from the verb resbalar, which literally means "to slip" or "to slide". However, in this song, Lola Indigo uses the very common colloquial phrase "a mí me resbala".
This expression idiomatically means "I don't care" or "it's like water off a duck's back." She sings, "Ahora me está llamando, a mí me resbala" (Now he's calling me, I don't care), perfectly capturing her indifference to her old crush. It's a sassy and powerful way to say that his attempts to get her attention simply slide off her without any effect.
This song is the ultimate glow-up anthem! Lola Indigo, TINI, and Belinda sing to a guy from their school days who completely ignored them back then. They ask him, "Soy aquella niña de la escuela / La que no te gustaba, ¿me recuerdas?" (I'm that girl from school / The one you didn't like, remember me?). Now that they're successful and confident, he's suddenly interested. They see him staring as they walk by, saying "Oh nena, oh nena" (Oh baby, oh baby).
But it's way too late for him. The singers are enjoying their success and have no time for someone who only wants them now. They tell him straight up: "Cuando pudiste, tú no quisiste / Y ahora que tú quieres, no se va a poder" (When you could, you didn't want to / And now that you want to, it's not going to happen). They've moved on, feeling more powerful and even a little dangerous: "Ahora me puse más buena, pero más mala" (Now I got hotter, but badder). They even throw in a great line telling him to get over himself: "Baja de la nube que no eres Goku" (Get off the cloud, you're not Goku).