Envolviste comes from the verb envolver, which literally means 'to wrap' or 'to envelop'.
In this heartbreaking song, Selena sings about how a deceitful lover wrapped her up in a beautiful illusion ("Me envolviste dentro de aquella linda ilusión"). She uses this word metaphorically to describe being completely surrounded and captivated by false promises, making it a vivid and poetic way to express a romantic betrayal.
Have you ever been promised the world, only to have it all disappear? That's exactly what Selena is singing about in "Cobarde." She lays out all the beautiful things her lover told her: that he'd come back soon, that he'd forget his ex, and that he wanted to be hers. He even said, "Que era yo la medicina para tu dolor" (That I was the medicine for your pain). She believed every word and gave her heart to him completely, "sin ninguna condición" (without any condition).
But now, time has passed, and he's nowhere to be found. He's forgotten her. Instead of just being sad, she's calling him out directly. "Cobarde, tú no eres más que un cobarde" (Coward, you are nothing more than a coward). She says he doesn't have the courage to even speak to her and is just running away. The song ends with a powerful warning: sooner or later, "Te vas a acordar bien de mí" (You are going to remember me well).