The poetic element you are describing is known as alliteration. Alliteration refers to the repetition of consonant sounds at the beginning of words that are in close proximity to each other. It creates a musical quality and can enhance the mood, rhythm, and overall effectiveness of a poem.
For example, in the phrase “She sells sea shells by the sea shore,” the repeated initial "s" sound demonstrates alliteration.
If you have any other elements of poetry you'd like to explore or need explanations on, feel free to ask!