The schwa sound is represented by the upside-down "e" symbol (ə) and is a very common vowel sound in English. It's often described as a short, unstressed "uh" sound.
In "exercise", the "e" in the first syllable is very short and unstressed, making it a schwa sound.
Here's how it breaks down:
ex-er-cise
The first syllable has the schwa sound (ə).
Let me know if you want to hear other examples of words with the schwa sound!