It's a literary device where words that start with the same consonant sound are placed close together. This creates a pleasing rhythm and emphasizes the words.
Examples:
* Peter picked a pickle.
* She sells seashells by the seashore.
* We went to watch the waters wash the whore.
Alliteration is often used in poetry, advertising slogans, and children's rhymes to make the language more memorable and impactful.