It is a reduced vowel, which means it is shorter and quicker than other vowels and only appears in unstressed syllables. This vowel is found in most unstressed syllables in English.
Grammar Tip: In addition to unstressed syllables within words, /ə/ is often used in a certain set of words called "function words" (or "structure words"). These are words that carry grammatical meaning within a sentence. They are often spoken quickly with reduced vowels in conversational English.