Alliteration is the repetition of two or more consonant sounds at the beginning of words in the same sentence, and it generally serves to enhance meaning or add emphasis to important words or ideas. It is generally used in poetry, but it can also be used in prose, of course.
Chapter five of The Pearl by John Steinbeck begins with an awful scene between Juana and Kino. She has stolen the pearl and they are about to physically fight over it. In the second paragraph, Kino follows Juana and we read this [bold print added by me to indicate the alliteration]:
...he could hear her quick footsteps going toward the shore. Quietly he tracked her....
In the fifth paragraph, Steinbeck writes about Juana after Kino has beaten her:
Juana, in her woman's soul, knew that the mountain would stand while the man broke himself; that the sea would surge while the man drowned in it.
One more. In the next paragraph, we read this one, describing Juana:
Her back was bent with pain and her head was low.
Notice that, in each of the examples above, the alliterative words are also the most significant words in the sentences, enhancing meaning by adding emphasis.
Examples of alliteration are easy to find, and I would encourage you to find some examples on your own, as well, since alliteration is a common literary device and you will probably be asked to identify it often in your studies.