An interjection is a part of speech that expresses emotion and is often used in exclamatory sentences. Interjections can stand alone or be placed before or after sentences. Common interjections include words like "Wow!", "Ouch!", and "Hey!" which are often followed by exclamation points. The document provides examples of interjections used in different situations like greeting a friend, cheering for a sports team, reacting to food or pain. Students are asked to fill in the blanks of sentences with the correct interjections from a provided list.