According to the prevailing values of white society, blue eyes are the epitome of beauty. This standard is everywhere, from movies to advertising billboards to story books. It's also displayed on the wrappers of a brand of candy bar that Pecola really likes. The little girl depicted on the wrappers, Mary-Jane, has blue eyes, and Pecola wants to be just like her.
Because of all the terrible problems she's been having at home and at school, Pecola thinks she's ugly and really hates the way she looks. Unable to find any beauty in her own ethnic identity, she tries to find it in the version of beauty perpetuated by white society.
Pecola has been so traumatized by the abuse to which she's been subjected at home that she's created a fantasy world as a kind of defense mechanism into which she retreats when life gets too hard for her. And in that world, where she increasingly spends much of her time, beauty means having blue eyes.
Pecola is so deeply immersed in this fantasy that she becomes convinced that, if only she too had blue eyes like the little white girl on the side of the candy bar wrapper, then everyone would regard her as beautiful and all her problems would suddenly disappear.