The answer above rightly identifies the refrain of this poem, "Where are we going, Rubee?" which is repeated throughout, and serves to underline the melodic quality of the verse. The refrain is a typical component of "spirituals," or songs sung by enslaved peoples to maintain a rhythm and also sustain morale, and the poem makes use of this. However, there are a number of other instances of repetition that also support the rhythm and "melody" of the poem.
We see both alliteration and consonance at the beginnings of lines, as in "Strong the Ghiblee wind is blowing / Strange and large the world is growing." The repeated str- sound creates a run-on connection between the two lines, seemingly pulling the one into the other. A similar effect of cohesion is created by the anaphora in "Here we thirst and here we hunger / Here the Moor-man smites in anger," where the "Here—" phrases continues over a span of two lines. The anaphora also has the effect of breaking each line into half-lines, or a-b lines. This is observed in other verse intended to be sung and learned by memory, rather than written, such as in Anglo-Saxon verse, where the half-line repetition contributes to the cohesion and rhythm of the whole.
This effect permeates the poem, particularly in its latter stanzas, with the repetition of phrase and form rising to its peak in the stanza beginning: "We are weak but Thou art strong." The "We are . . . but Thou" construction is repeated over and over in such a way as to reinforce the message: the weakness of the slaves is continually compensated for by the strength of a higher power. This stanza is a sort of crescendo leading towards the climax of the poem.