Consonance:
Repetition of a consonant sound within two or more words in close proximity.


Example:
"A Quietness distilled
As Twilight long begun,
Or Nature spending with herself
Sequestered Afternoon."
-Emily Dickinson


Explanation: 
The example of consonance in this poem is the repetition of the letter 'n'. The purpose of this is to get the desired rhythm and feeling that Dickinson wants the reader to have. It creates a good rhythm by not rhyming the words, but instead having a slant rhyme. This gave Dickinson more freedom and flexibility when writing the poem, and it also gives the reader more freedom to interpret the tone and rhythm better.