An ELT Glossary : Vowel weakening (or reduction)

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).

Look at the sequence I ex'pected to have been 'there 'earlier.  Between the stressed syllables pect and there, there are four unstressed syllables :  -ted, to, have and been. Pronounced in their citation forms, these would be  /ted/,  /tuː, /hæv/, and  /biːn/. But connected speech their pronunciation is likely to become /tɪd/ /tə/ /əv/ /bɪn/. In each case there has been a change in the vowel to a weaker sound which can be pronounced more quickly - /e/ and /iː/ have changed to /ɪ/, while /uː/ and /æ/ have changed to the schwa /ə/.

These aren't the only changes that are made, of course. In have the /h/ has been elided (and the same thing might well happen to the /v/). But vowel reduction, very often involving the schwa, will nearly always occur in a string of unstressed syllables. The words affected are very often grammatical words (auxiliaries, articles, prepositions etc) and morphemes as here, but the same thing will happen with the vowels in the unstressed syllables of ordinary words. Compare the pronunciation of the syllable man in the first sentence below (where it's in a stressed position) and in the second (where, as the second half of a compound noun, it's unstressed).

Look at that man!             /'lʊk ət 'ðæt 'mæn/
There's the postman!      /'ðeəz ðə 'pəʊsmən/