It is the most common vowel sound in English and is represented by the symbol: /ə/
Example
The word 'banana' has a main stress on the second syllable and the other two syllables are schwa sounds.
In the classroom
Many teachers and learners feel that correct pronunciation of the schwa is fundamental in developing a satisfactory accent, since it is found in many weak forms of words, and in unstressed parts of words. Learners often have great difficulty reproducing the sound.
Further links:
https://www.teachingenglish.org.uk/article/phonemic-chart
https://www.teachingenglish.org.uk/article/teaching-schwa
https://www.teachingenglish.org.uk/article/rhyming-blockbusters