"The Trials of Phillis Wheatley" is focused on both the eponymous woman herself and the creation of African-American literature in America. Henry Louis Gates Jr. gave the speech version of the text at the Library of Congress in 2002.
Phillis Wheatley was an enslaved woman purchased by a family in Boston during Colonial times in America. She was intelligent and the family worked to educate her; she was a published poet and the first black person to publish a book of poetry (which she did in England). However, not everyone believed that a black woman could be a poet. Some people thought she was an imposter. She was called to a trial so that they could find the truth of her writing. Even Thomas Jefferson said that she was likely more of a plagiarist than a poet because religion could allow her to be who she was but not allow her to be a poet. (Many of her poems were influenced by her religion.) Ultimately, though, the panel of judges said that she did write her poems.
The real question wasn't whether Phillis herself was capable of writing poetry. It was whether black people in America were capable of it. It was whether they were as intelligent as white people. Gates shows that some of the founding fathers and many of the people in Colonial America and abroad didn't believe that black people had the same intellectual and artistic capacity as white people. They didn't see them as full people with creative souls. Phillis's writing ability was something that proved to some of those people that black people had the same types of talents and capabilities; their prejudices were what convinced them that she was a plagiarist.
In later years, Phillis was largely excluded from the ranks of other important black American writers because she wasn't seen as being black enough. They said that her experience didn't reflect that of other black people in America during postcolonial slavery. (This could have been because she was freed or because of the way she expressed her thoughts about being brought to America as a slave in a positive light.) They said she didn't condemn slavery enough. Gates argues that her writing is important and that she belongs in the ranks of other influential black American writers. He says that she—and Thomas Jefferson, whose racist attitudes toward black artists were that they were incapable of real, creative art that white people could produce, challenged black people to prove him and similar people wrong—shaped and influenced the course of black literature in America.