Vowels are speech sounds made with an open vocal tract, without the airflow being blocked by the tongue, teeth, or lips. In poetry, vowel sounds matter because they shape rhyme, assonance, rhythm, and the overall music of a line. The letters a, e, i, o, and u often represent vowels in English, and sometimes y does as well.
Example
The moon moved low
through blue June air,
soft o and u sounds
lingering there.