What you are referring to - the main point - is also known as the theme of a story. There can be many themes in a story, or many messages that an author expresses while he/she writes. Some are more developed than others, and so are considered the "main" themes.
This book deals with racism, nationalism, personal integrity, motherhood, personal identity.. just to name a few. One of the "main" themes, though, is the need for acceptance. Every human being needs to feel that he/she is accepted in some part of the world. Angelou travels to Africa searching for a place where others would look as she did and feel as she did. She wanted to be free from discrimination and to be a central part of a society, not a minority.
However, what she experiences when she arrived is another theme - that discrimination exists everywhere. Angelou realizes that the Ghanians are protective of their land and their own community. She is an interloper, and thus can not be a central figure in their already established society. They resent her assumptions and assume that she can not understand them and their ways. This resentment even makes Angelou question the causes of discrimination in America:
"Had what we judged as racial prejudice less to do with race and more to do with our particular ancestors' bad luck at having been caught, sold and driven like beasts?"