Alliteration: 
The repetition of the initial consonant sounds in two or more neighboring words. The repetition can reinforce meaning, unify ideas, and/or supply a musical sound.


Example: 

Rain

Rain races onto the porch
Hitting hard like
Rockets rushing in a race to the glass.
Rattling rain, rattling the window panes.
The droplets dance daintily
Until they cleverly connect in
A pool of promise for a rainbow.

                         -Poetry by Natasha Niemi

Explanation: 
Alliteration gives poetry a unique sound, emphasizing on specific phrases and represent the action that is taking place. In the example the alliteration helps the reader see the action of the rain, by the constant repetition of alliteration in the description of the rain. The alliteration creates imagery in the readers mind, and also lets them see the tone and mood of the poem. Alliteration is useful in helping create a lyrical tone, and emphasize the action and imagery throughout the poem.