"Cross" begins with a simple statement about the race of the speaker's parents. Right off the bat, one meaning of the poem's title becomes clear: the speaker is a "cross" between a white man and a Black woman.
The poem's immediate focus on the speaker's biracial identity places the subject of race front and center. But while these first two lines communicate the races of the speaker's parents (and by extension, the race of the speaker), they don't tell readers much else. Where and when does the poem take place, readers might wonder, and what's the speaker's gender? What kind of relationship does the speaker have with their parents? For now, these things are unclear. This suggests just how important race is in the speaker's world: it apparently supersedes other considerations.
By focusing so exclusively on race, the speaker also sets up a stark contrast, or juxtaposition, between their white father and Black mother. The parents are not only different races but are also discussed in separate lines of the poem. This separation on the page subtly emphasizes their different experiences. In fact, the only things connecting them are the adjective "old," which the speaker uses to describe them both, and the fact that both of them are responsible for the speaker's existence. The difference—and distance—between the speaker's parents will be important later in the poem, when the speaker thinks about the dramatic inequality between white and Black people in this society.
Finally, let's examine the meter of these lines:
My | old man’s | a white | old man
And my | old moth- | er’s black.
The first line contains a single stressed syllable ("My"), The second line has just three iambs, so it contains one less stressed syllable than the first line.
This looks a lot like common meter, a pattern of alternating lines of iambic tetrameter and iambic trimeter. Iambs are metrical feet that follow an unstressed-stressed syllable pattern; iambic tetrameter means there are four of these feet per line, while trimeter means there are three.
Common meter is often used for a special type of stanza called the ballad stanza (spoiler alert: the three stanzas of "Cross" are all ballad stanzas!). The alternation between four- and three-beat lines creates a highly musical sound that's good for both telling long stories and conveying intense emotion in short lyrics, such as "Cross."
Of course, these lines don't fit common meter perfectly: line 1 is missing a beat, and it's possible to read extra stresses onto "old" in each line. These lines are a good example of the way Langston Hughes subtly experimented with traditional form and meter in order to make his poems—"Cross" included—more musical and expressive.