It's represented by the upside-down "e" symbol (ə) in phonetic transcription.
Here's how it breaks down:
* rib /rɪb/ - The "i" here is a schwa sound.
* bon /bɒn/ - The "o" here is a different vowel sound.
The schwa is a very common vowel sound in English, and it often occurs in unstressed syllables.