This is a rather broad question, but I'll focus on the contributions of a few major African American women writers. Generally speaking, one of the many contributions made by African American women writers has been the way they have presented realistic and sympathetic portraits of the lives of average African American women, men, and families.
There are not many well-known African American women writers of the colonial and antebellum period, but two of these early writers are important to mention. Phillis Wheatley is the first recognized African American female poet. She wrote poetry that heavily referenced religion and faith but also subtly questioned whether African Americans should be considered inferior in the eyes of God and of Christians. Harriet Jacobs was a former slave who wrote an important slave narrative entitled Incidents in the Life of a Slave Girl. This was the first American text to give readers an inside look at the specific struggles undergone by female slaves. Jacobs presented the fear and shame associated with being sexually harassed and raped by slave masters and hated by their wives. She presented the terror of a mother potentially being separated from her children and helped white female readers to begin to understand how difficult it is to try to be a good mother under such an oppressive system.
Later African American women writers had different experiences, living and composing in the aftermath of emancipation and into the twentieth century. Zora Neale Hurston is an important writer of the Harlem Renaissance, whose novel Their Eyes Were Watching God has become a touchstone of feminist literature, as well, due to its portrayal of Janie Crawford's evolution into a confident, independent woman. Hurston also wrote the essay "How It Feels to Be Colored Me," in which she expresses her attitude toward racial discrimination in the early twentieth century and celebrates her unique identity. Hurston's work feel into obscurity for some time before her work was highlighted by another significant writer, Alice Walker. In addition to bringing attention back to Hurston's work, Walker wrote The Color Purple, a foundational text of twentieth-century African American literature that was both a critical and popular success. Walker presented Celie's struggles to overcome early childhood sexual abuse and later domestic abuse, as she, with the help of the strong women around her—Shug and Sofia—grows into a confident, self-assured, wise heroine.
Finally, one of the essential American authors of the twentieth and twenty-first centuries, and also a powerhouse of women's African American women's literature, Toni Morrison is also a Nobel Prize winner. Her novels poignantly portray the everyday tragedies of African American life, touching on the struggles of poverty, generational conflict, and the lasting impact of slavery and racial inequality in the United States. Morrison is also a literary innovator, tapping into postmodern and magical realist techniques in her masterpiece Beloved.
All of these women writers have shaped the American literary canon by bringing to the forefront the experiences of African American women and families, by incorporating realism and dialect into their works, and by continuing to innovate in terms of theme and style.