Vowel harmony applies to vowels in suffixes. When a suffix is added to a word, its vowel changes depending on the last vowel in the word.
We may say that the realization of vowel harmony is somewhat similar to different pronunciations of some English suffixes. Depending on the preceding sound, for example, the past tense suffix -ed in played, finished and started, or the plural suffix -s in cats, dogs and houses are pronounced differently.
In Turkish, however, different pronunciations are nearly always shown in writing.