The inferior frontal gyrus is a part of the frontal lobe. It's most important role is the control of the vocal tract through the motor cortex, primarily the movements of the mouth and tongue. It also plays a role in language comprehension.
Damage to the area manifests as Broca's aphasia, a condition where, in more mild cases, the patient cannot use grammatical helper words or complete sentences and, in more severe cases, the patient may only be able to say one word at a time, and then only with great effort.