What does the schwa sound mean in spelling?

Answer 1

Scwha indicates any vowel which is not "voiced" - it will generally be pronounced "uh": e.g. the "e" in "answer"; the "o" in "history"; the second "a" in "vandal".

The vowels are "voiced" because all vowels are voiced, but they are in an unstressed syllable.

Answer 2

The schwa sound is a neutral vowel sound commonly represented by the upside-down e symbol (ə). It is the most common vowel sound in English and is often found in unstressed syllables or in weak syllables.