This poem is about the violence instigated by racism in American history. The grandson essentially gives a brief biography of his grandfather and this is framed in the historical context of racism in the 20th century: the grandfather's lifetime. One thing to consider in this poem is the link between history and the individual. In the opening lines, the grandfather's neighbors tried to burn his house and this was inspired by the movie The Birth of a Nation. This film was provocative because of how it portrayed African-Americans and it is credited by some to have influenced a revival of the Ku Klux Klan. The grandson then briefly describes his grandfather's life as it is inextricably linked to the culture of racism throughout this period of American history. The grandfather is a part of this nation but has had to struggle against it as well.
One of the most important symbolic concepts is the analogy between the American nation and cancer:
he breaks on his set teeth,
stitched up after cancer,
the great white nation immovable
Here, the cancer is called a "white nation" and this is clearly a link to the racist members of white Americans. Both "nations" (racist part of America and the cancer itself) are inextricably linked to the grandfather. Racism is like a cancer. The grandfather has had to deal with racism and cancer because they are both part of his life. Racism is part of the nation that he lives in and cancer is part of the body he inhabits.
The grandson begins to understand his grandfather and this period of history as if he were watching the film (of history and/or his grandfather's life) played backwards. This notion of the film being played backwards also suggests that racism is still a problem.