An action verb describes an action, event, or state of being and is always found in the predicate of a sentence. There are two types of action verbs: physical action verbs that describe observable actions like writing, cleaning, and kicking, and mental action verbs that describe thoughts or feelings like knowing, believing, and wondering. The document provides examples of sentences using both physical and mental action verbs and encourages readers to listen to a song, understand its message, and do a related activity.