Langston Hughes attended college "on a hill above Harlem," where he was the only black student in the class. Thus, when he is assigned a theme to be written on a paper which is to "come out" of him, Hughes wonders if his professor will know what is true for him, having had much different experiences from her. Yet, there is some common humanity between them "That's American," even though there are times when he does not always wish to be a part of the professor.
But we are, that's true!
As I learn from you,
I guess you learn from me--
although hyou're older--and white--
And somewhat more free.
In the end, Hughes surmises that the teacher learns from him, as he learns from her, although she is older and white and "somewhat more free." So, while they share some experiences, others are unique to him and others obstacles for him.
For me, the author is trying to say that Americans are all tied together, regardless of what race they are. He is saying that black people are part of white people and white people are part of black people, whether each side would like it that way or not.
In addition, the author is trying to say that there are, in our society perceptions about people of different races. We think that people of other races are in some way fundamentally different from us. Hughes seems to be saying that this is not how things should be, but, at the same time, he realizes that that is how they are.