What are the major political underpinnings behind Maya Angelou's "I Know Why the Caged Bird Sings" (the poem)?

In Angelou's poem "I Know Why the Caged Bird Sings," a free bird floats downstream and "dares to claim the sky," while the caged bird has clipped wings and tied feet and paces in his cage. The caged bird sings "of things unknown/but longed for still." In other words, the caged bird dreams of what he longs for but can't achieve; as Angelou writes in the last line of the poem, "the caged bird/sings of freedom." 
The political underpinnings of the poem are the inequality between different people in the United States. Racism, sexism, and other types of discrimination mean that some people can achieve a great deal of freedom over their own lives, as the free bird in the poem does, while others who are in groups that face discrimination are metaphorically caged, in that they are not able to go where they want and achieve what they dream about. The caged bird sings of freedom in the hope that his song, which can refer to the protests of oppressed groups, leads to his release.