The meaning behind Stargirl "being erased" refers to her condition and goal of meditation. Despite the fact that Stargirl has a distinctive identity and sense of self, her desire in meditation is to overcome her sense of self and become unified with the powerfully transformative force present in the universe:
I’m erased. I’m gone. I’m nothing. And then the world is free to flow into me like water into an empty bowl…. And… I see. I hear. But not with eyes and ears. I’m not outside my world anymore, and I’m not really inside it either. The thing is, there’s no difference between me and the universe. The boundary is gone. I am it and it is me. I am a stone, a cactus thorn. I am rain. I like that most of all, being rain.
Part of the reason that Stargirl is so different is because she is operating on a different level of self actualization. It is one in which she is no longer concerned with temporal reality. In her meditation, she strives to "erase" her identity, her sense of individual self. This enables her to become "one" with the world. Her "erasing of self" is one in which there is no "boundary" between the individual and the force of universal spirituality that is present. When Stargirl "erases herself," she is able to "be" in the universe as a part of a larger force and not as an individual.
It is for this reason that Stargirl is who she is. While she can be seen as quirky and unique, the reason for this is that she no longer clings to a sense of self in the traditional sense. Her identity is submerged into something larger when she meditates. Her meditation gives root to who she is and what she embraces. It is this condition that enables her to not care what others think about her. The significance of her "erasure" enables her to exist on a transcendent level, and move past one of contingent values such as social acceptance and social mobility. Leo cannot "erase" himself and it is for this reason why his attempts at meditation fail.