In Richard Wright's auto-biographical novel, Black Boy, Wright recounts a time that he worked a job outside places of business as a young man. When white people came by, Richard would look at them, making eye contact; however, after observing Richard doing so, a friend tells him that he will be fired if he keeps looking at these people. He further instructs Richard to shuffle and smile and laugh and act "happy-go-lucky" because this is the image that white people like. In other words, his friend encourages Richard to "wear the mask" of joviality that white people desire on the blacks.
Much like this passage from Black Boy, Dunbar's poem expresses the facade that blacks of his time and especially the slaves were forced to present to the white world. "This debt" owed to the world in order for them to survive is often not a complete mask, but has "myriad subtleties" hidden in it. That is, while they are polite and obsequious to the whites that they serve, the African-Americans may inject a quick sneer or a slight word that subtlely suggests anything but contentment or agreement with the situation in which they find themselves.