When vowels are produced incorrectly, that makes speech very difficult to understand.
There are three types of vowels. Front vowels, back vowels, and mixed vowels. There are also diphthongs which are words that are two vowel sounds in one syllable. The vowel sounds are different from each other because you change the shape of your mouth when making each sound. Some are made in the front of your mouth with the mouth closed ( i.e. read). Some are made in the center of your mouth with your mouth midway closed (i.e. but) and others are in the back with your mouth open (i.e. root).