What is the meaning of the last lines in the novel?

In the novel's conclusion, Esperanza vows that "one day I will pack my bags of books and paper. One day I will say goodbye to Mango. I am too strong for her to keep me here forever." In these lines, the author reveals Esperanza's true reason for wanting to move far away from Mango Street: so that one day she can come back for those- like so many of the characters we have met- who cannot make it out on their own.