What is the meaning of stress syllables?

A stressed syllable of English has more prominence than an unstressed syllable, because it is louder, longer, or has higher pitch. The vowels of unstressed syllables may be reduced to schwa or bar-i, but the vowels of stressed syllables are not reduced. Stress comes in degrees, the most prominent being "primary stress", then of lesser prominence: "secondary stress", "tertiary stress", and sometimes lower degrees of stress.

In the history of English, the stressed long vowels of Middle English underwent the connected series of changes called the "Great Vowel Shift", but the unstressed vowels were not shifted.