The first line repeats the poem's title and introduces its extended metaphor. The mask, which represents the false emotions a person might "wear" in front of other people, both smiles and lies—which indicates that the outward appearance of the mask does not match the true emotions that are beneath it. In other words, the people who wear this mask aren't smiling, and aren't happy. While readers don't know exactly who these people are, it's already clear that they must for some reason hide what they're thinking and feeling from the rest of the world. The phrase "We wear the mask" will also repeat two more times in the poem as a refrain, never letting the reader forget the burden this group carries.
The first line also introduces the speaker, who uses the pronoun "we" throughout. The speaker is not necessarily an individual, then, but rather a representative of a larger group. Though subjective, readers could even potentially include themselves as being a member of this "we"—and maybe empathize with the struggles of the community being described.
The second line continues to describe the mask as covering the entirety of the speaker's face, demonstrating that the mask fully obscures any features which could give away the speaker's true emotions. In particular, note how the mask casts a shadow over the eyes of those who wear it. Eyes are often used in poetry (and literature in general) to represent a person's truest self—think of the phrase "eyes are the window to the soul." The fact that this mask "shades" its wearers' eyes emphasizes how much the wearers must hide not just what they're feeling, but who they are.
These lines also introduce the poem's meter, which is a pretty steady iambic tetrameter—meaning most lines have four stressed and four unstressed syllables in the following alternating pattern:
We wear the mask that grins and lies,
It hides our cheeks and shades our eyes,—
Iambs mimic the way people tend to speak in regular English, lending the poem a casual, colloquial feel. The focus here isn't on fancy or overly poetic language, but rather on conveying a clear message to the reader—something the poem's straightforward meter reflects.
Readers will also note the clear, perfect rhymes between lines 1 and 2 ("lies" and "eyes"), which mark the beginning of the poem's Rondeau rhyme scheme (something we'll talk about more in "Rhyme Scheme"). What's important to know here is that these AA rhymes will repeat in every stanza of the poem going forward, creating a sonic echo and sense of consistency that ties the poem together.