An interjection is a word that expresses emotion and is not grammatically related to the rest of the sentence. Common interjections include ouch, wow, ahem, boo, eww, oops, whoa, and yahoo. Interjections can be used at the beginning, middle, or end of sentences to convey surprise, approval, dislike, or other emotions. The document provides examples of interjections and their meanings, and instructs the reader to complete an activity using interjections in their own sentences.