Here's how it works. Azeri uses suffixes to create cases and to change the meaning of words. Whenever a suffix is added to a word, the vowel in the suffix has to agree or harmonize with the last vowel in the original word - that is, the last vowel in the word must agree with the first vowel in the suffix.
If the suffix contains only two first vowels, they will be ə and a. For example -lar and -lər to make a word plural. This type of suffix is represented in Azeri grammar as ə squared. These are the vowels that harmonize:
If the suffix contains four first vowels, they will be i, ü, ı, and u. For example, -si, -sı, -sü, -su. This type of suffix is represented as i to the power of 4. These are the vowels that harmonize: