The narrator of the poem is a young mulatto, expressing his frustration at being both black and white but never fully belonging to either of the two races. He is not accepted by blacks because he is half white, and he is shunned by whites because he is half black. The historical significance of this is that, during the 1920s (in the U.S.), the cultural and racial segregation of African Americans and Caucasians were very defined and apparent. As such, the narrator is stuck in a sort of purgatory/gray area/limbo, for he is deprived of the chance to be immersed in either race due to racial pride. Thus, he is left in a state of confusion, suffering an identity crisis. He has no one with which he can identify.
As the narrator matures, he realizes that he can't blame or hate his parents for who he is. It doesn't make any sense. At the end of the day, he'll still be who he is, so why hate?
The last stanza emphasizes the poverty of the Blacks versus the wealth of the Whites during this time period. For, the narrator says that his father died in a big house while his mother died in a shack. He still doesn't know how to categorize himself in terms of identity, but he accepts the issue as it is. Once he decides which path to lead--life as a black man or life as a white man--then he'll know where he'll die.