The poem sets its scene (establishes its "Tableau") with the image of two young men crossing a street together. One is "black," the other is "white," and they are "Locked arm in arm." The street might be in any town or city, but the casual term "the way" (as in a phrase like "just down the way") makes it sound like part of a familiar neighborhood. Perhaps this setting is familiar to one or both young men, or to the speaker describing the scene.
"Tableau" was published in the U.S. in 1925, during an era of extreme and institutionalized racism and homophobia. For example, Jim Crow segregation laws were still in effect throughout much of the country and would be for several more decades. In that context, the public display of affection described here would have seemed dramatic and provocative, both for the community in the poem and for readers of the poem. The young men's display of interracial, same-sex affection openly defies taboo. (Some readers in 1925 would have read their affection as homoerotic, while others would have read it as "just friendship," but all American readers would have understood that it violated social codes surrounding race.)
The poem is written in common meter, meaning that it alternates between lines of iambic tetrameter (eight syllables following an unstressed-stressed or "da-DUM, da-DUM" rhythm) and iambic trimeter (six syllables following the same rhythm). However, in lines 1-2, there are two significant variations in this pattern:
Locked arm | in arm | they cross | the way,
The black | boy and | the white,
The first metrical foot of line 1 is a spondee (stressed-stressed) rather than an iamb (unstressed-stressed), while the second foot of line 2 is a trochee (stressed-unstressed) rather than an iamb. The result is that both "Locked" and "boy" are stressed where they normally wouldn't be.
The stress on "Locked" emphasizes how intimate and tightly linked this pair is. The stress on "boy" ensures that both syllables of "black boy" are emphasized, contrasting this phrase sharply with "white" (also a stressed syllable). The alliteration of "black boy" and "way"/"white" further emphasizes the words "black" and "white." These emphases signal that race (both racial division and cross-racial bonding) will be a central theme of the poem.