It's represented by the symbol ə, and it's a very common vowel sound in English. It's often described as a "short, unstressed" vowel sound, and it appears in words like:
* about (əˈbaʊt)
* the (ðə)
* again (əˈɡɛn)
* taken (ˈteɪkən)
* supply (səˈplaɪ)
Let me know if you have any other questions!