Barb wire fences use pain to keep animals from crossing into other areas. Wooden fences keep people from seeing what is beyond them. Fences of brick or masonry utilize their strength to keep people where they are.
All literal fences create boundaries.
They may inflict pain, limit sight, or seem impenetrable. Humans build the same kinds of fences with each other. Morrison uses the two characters to illustrate the way in which race causes separation between people. These fences are created because of differences in appearance. Roberta and Twyla look different.