
Originally Posted by
tdol I'm not sure that I get what it means too clearly, though.

From the root vowel, we can predict what the affix vowel will be. For example, if the root vowel is /o/ (e.g,
son "end") and the affix vowel is /i/ (e.g, -
in genitive singular), then the affix vowel /i/ will share the same place of articulation, with respect to roundness and/or backness, with the root vowel /o/, like this,
Turkish data
Root:
son
Genitive Singular Affix: -
in
==> s
on
un
Note, the affix vowel /i/ is pronounced as "u", a round vowel or a vowel produce with rounded lips. Both "u" and "o" share the articulatory feature [+round]. That's vowel harmony.
If, say, the affix vowel is /e/ (i.e. -
ler "genitive plural), then /e/ will share the same place of articulation as the root vowel /o/, like this,
Root:
son
Nominative Plural Affix:
-ler
==> s
onl
ar
Note, the affix vowel /e/ is pronounced as "a", a non-high vowel. Both "e" and "a" share a similar articulatory feature. They are produced with the tongue situated in the lower part of the oral cavity (mouth).
The root and affix vowels are said to be in harmony because they share similar features. The speaker modifies the affix vowel so that is shares articulatory properties or harmonizes with the root vowel.
SOURCE
http://www.u.arizona.edu/ic/heiberg/turkish/data.html