Assonance is a kind of internal rhyme that makes use of repeated vowel sounds. The vowel sounds are woven together to create a cool sonic effect. It's a trick of the trade that poets use to create and enhance meaning.
Want more? Check out assonance in Notorious B.I.G.'s song "Juicy", or our discussion of assonance and consonance (kind of like assonance but with consonants) in Gerard Manley Hopkins's poem "Spring".