Rudyard Kipling's "The Stranger" is a poem that makes quite uncomfortable reading to the modern reader—Kipling is describing the supposed difficulty of understanding people who come from a different culture, with different beliefs. The final stanza of the poem is usually interpreted as a warning against racial mixing—the suggestion being that unless "the grapes be all on one vine," the result may well be "bitter."
This final stanza contains a good deal of metaphor, but that's not what you've been asked to look at, so: let's consider alliteration first.
Alliteration is the repetition of the same initial letter in several words placed close together. We don't actually see a lot of this in the poem, so it's notable that Kipling alliterates twice on the word "bitter"—we see "bitter bad," and "bitter bread." This underscores the word, the bitterness, and draws our attention to it. We also see alliteration on "them and their," which again draws attention to this and to the idea of otherness it generates.
Assonance is similar to alliteration, but it means the vowel sounds are echoed from one word to another. One example follows on from "bitter bad"—there is assonance on "least," "hear," and "see." This creates a unity between these concepts, suggesting that seeing and hearing the same things are especially important.
A simile is a figurative device in which one thing is compared to another—something is "like" something else. The word "as" might also be used. There actually aren't any similes in this poem, although there are metaphors, as noted, in the final stanza.