An ELT Glossary : The schwa
The schwa is the vowel sound symbolised in phonemic script by /ə/. It is common in unstressed syllables whether these are unstressed because of their normal position in the word, as the syllable man in postman in the second sentence below (compare it with the word man in the first sentence), or because they are grammatical words (auxiliaries, prepositions, articles etc) which fall in an unstressed position in the sentence - compare the two different pronunciations of has in the sentence below, stressed in the first sentence and unstressed in the second.
That man has forgotten to pay! /'ðæt 'mæn həz fə'gɒtən tə 'peɪ/
Has the postman arrived yet? /'hæz ðə 'pəʊsmən ə'raɪvd 'jetˈ/
Look at all the words in the two sentences and you'll see that in the unstressed syllables, the schwa always occurs. These sentences were, however, constructed to exemplify it. It's not the only vowel that can occur in unstressed syllables but it is certainly far and away the most common.
Why does this happen? English is a stress-timed language - ie the rhythm is determined not by the total number of syllables in the utterance but by the number of stressed syllables it contains. The stressed syllables are pronounced at (fairly) regular intervals, with the unstressed syllables being "squashed in" between them. Whether there's one unstressed syllable between two stressed syllables, or two of them, or three or four, they will be pronounced in roughly the same time.
This inevitably means that "something has to go" - and gives rise to a number of features of connected speech, one of which is vowel weakening (or vowel reduction). And, as was said above, the most commonly occurring weak vowel in English is the schwa.