The alphabets a, e, i, o, u are known as vowels whereas the remaining alphabets of English Language are known as consonants. According to the definition given in the Cambridge Dictionary, "<i style="">a vowel is a speech of sound produced by humans when the breath flows out through the mouth without being blocked by the teeth, tongue, or lips"
We use 'an' before words starting with a vowel. For example - an apple, an umbrella, an incident etc.
A vowel has its importance in English Language as no word can be formed only by using consonants. For example - the word rsnnc has no meaning and no tone, but on adding the desired vowels it forms a word - resonance. Thus vowels gives the syllable or word its meaning