You've never heard of the schwa?
It is the most used vowel in the English language. It's the small, soft "uh" sound you often make in place of any other vowel when a syllable is unstressed. Just totally relax your mouth, let your jaw fall open a slight bit and say "uh". You will sound like a cave person considering speech for the first time. Needless to say, the schwa makes spoken English difficult to understand for an ESL learner.