First introduced in the title (which also acts as a refrain), the extended metaphor of the mask serves as the backbone of the poem. The speaker explains the meaning of the mask to the reader in the first stanza, illustrating that this metaphorical mask is a tool used for deceiving others. As the speaker continues, the mask is revealed to be more than a simple con, meant to trick others out of amusement—more specifically, this mask is used to hide its wearers' pain and suffering.
Of course, the group the speaker is a part of doesn't literally wear a mask over their faces at all times; instead, the mask represents the fact that these people have to constantly calibrate their expressions to appease an oppressive, prejudiced society. Members of this marginalized group cannot show their true faces to the world without risking repercussions.
The use of extended metaphor regarding the mask allows the reader to understand the many intricacies of what it means to be oppressed. The mask is initially presented as a forced tool of deception that eventually blossoms into a point of pride: in stanzas 2 and 3, the speaker seems to adopt a tone of defiance while discussing the mask, suggesting that the speaker (and the group associated with the speaker) has begun to celebrate their own ability to survive in such an unjust world.
Overall, then, the mask is a tool of survival and thus representative of the pain of oppression, but it is also a symbolic representation of the speaker's endurance and resilience. When the speaker exclaims, "We wear the mask!" in the final line, the reader is able to understand that the exclamation is one of anger, defiance, exhaustion, and celebration all in one.