/ə/ The schwa sound is a lazy vowel and the IPA symbol for it is an upside down lower case letter 'e'. It is a soft, neutral replacement for the other vowels. Although not always taught, the schwa sound is the most common English vowel. Long ago, in Old English, vowel letters represented their own sounds. However during the Middle English period (1066 to 1485) vowel pronunciation varied and changed. Although the schwa sound was created by lazy pronunciation centuries ago, today it is a necessary part of English pronunciation.
The schwa sound is most often spelled with the letter 'a'. For example: About, America, Russia However other vowel letters are used occasionally. For example: water, activity, apron, cactus and pyjamas. There are also replacements for long vowels, as in 'mountain'. In general the vowel in an unstressed syllable is often a schwa sound. They also are very common in grammatical function words. Examples: a, an, the, of, about...
The vowels (a, e, i, o, u, oo, ə) are created by the mouth not stopping the air pushing through it. The other letters, consonants, block or partly block the air with the lips, tongue or teeth. Every English syllable has at least one vowel sound. The consonants can't even be pronounced properly on their own and need a vowel for company. Think of the apparently vowel-lacking acronym MTR. It is pronounced, short-e sound, m-sound, t-sound, long-e sound, ah-sound, r-sound.
Second language speakers often make one of two schwa-related errors. The first common error is to pronounce the short vowel sound for the letter used, instead of the schwa sound. This error makes the person's speech sound odd, but understandable. Consider the word 'China' with a short-a sound compared to 'China' with the correct schwa sound. The second common error is much too common in Hong Kong. Some people use the schwa sound for every short vowel! People speaking in this way are very difficult to understand, even for old China hands who are accustomed to hearing it. Foreigners would not understand English with the wrong vowel sounds at all.
It is critical for children learning English to be able to clearly separate the short vowel sounds and not use a lazy sound when it is not correct. Not recognising the schwa sound can cause problems for children when they try to match sound and spelling. They will often substitute the wrong vowel when spelling. (about vs. ubout) The lazy vowel is far from being a lazy topic, spelling English words when the letters do not always match the sounds is challenging.