Assonance

Definition: In poetry, the repetition of the sound of a vowel or diphthong in non-rhyming stressed syllables
Examples:

The Bells
by
Edgar Allan Poe

Hear the mellow wedding bells,
Golden bells!
What a world of happiness their harmony foretells!
Through the balmy air of night
How they ring out their delight! 

 Significance: The important is to mess with words and have fun with it.