Black American women are subjected to multiple forms of devaluation: because of their race, because of their gender, and because of the intersection of the two. Intersectionality is a term coined in the early 1990s by Kimberle' Crenshaw and is used to mean the intersection of two or more aspects of a person's identity that may show up or be presented in any given moment or situation.
Overall, women in American society are treated as less than men. This may be demonstrated by getting less pay for equal work, getting less exposure in professional fields, men getting credit for women's ideas, getting less air time in entertainment fields, or many other ways. Black women are no exception.
Focusing on colonization as an umbrella, we can say that European men objectified African and African American women in many ways. To use slavery as an example, black women's holding compartments on the slave ships were often right below the captain and crew's quarters. This was so the white men could have easy access to the black women for when the men wanted to rape them. This practice carried over into the actual institution of slavery, where similar arrangements were made for the slave masters and overseers to have their "favorite" women nearby so that they could use the women sexually—that is, so they could rape or otherwise harm the women at will. Black people were considered the property of slave owners. To be classified in such a way is to be objectified, or turned into an object. Black women could then be used without their consent, with severe consequences if they resisted. During this time, black women had no agency—the right to choose what happens to them. A table, for example, is an object without agency because it cannot choose what will happen to it; black women were used in a similar way.
Another example of being an object—in this case meaning less than human—is the characterization of black people as being animals or animal-like. The racist remarks directed toward Michelle Obama and Serena Williams serve as a modern-day example of this type of comparison. There are articles, blog posts, YouTube videos, and other kinds of media that portray black women as apes, and this racist characterization creates a less-than-human paradigm through which to classify another.
The intersectionality of black women's gender and race makes them doubly subjected to the nasty perceptions of both women and black people. In fact, because the negative perceptions may even be compounded (taken together to become greater than the sum of their parts), getting along in the world is likely more difficult for black women.