We wear the mask that grins and lies, It hides our cheeks and shades our eyes, — (1-2)
The masks associated with race are so pervasive that they make it nearly impossible to honestly express, well, anything in Dunbar's poem. Those "cheeks" and "eyes" are the real instruments of our identity and if they're "hidden" we can't expect to have any sort of honest conversation.
And mouth with myriad subtleties. (5)
We get the sense that there's something mechanical about this line when we hear the word "mouth." The people speaking aren't saying anything honest because their racial circumstances prevent them from feeling and speaking as someone fully human in Dunbar's poem.
Nay, let them only see us, while We wear the mask. (8-9)
It's easier to just accept the mask for what it looks like, rather than take a peek at what's really behind it. In doing so, no one really sees the truth and the problems of racial prejudices continue to persist in "We Wear the Mask." Sad.