An enthusiastic lover of language would probably argue that every rule in the English language is essential. But when it comes to essential appositives, the argument is self-evident. It helps that there is also something known as a nonessential modifier. When you compare the two side by side, you can easily see the difference between the two.
Appositives Amplify Nouns and Phrases
An appositive is a noun or a noun phrase that adds descriptive meaning to a noun or a pronoun in a sentence. A nonessential appositive is just that -- it is not essential to a sentence. In other words, a nonessential appositive can be removed from a sentence without significantly changing its meaning. An essential appositive pulls greater weight -- it is essential to the meaning of a sentence. If you remove it, you change the meaning of a sentence, or at least diminish a reader’s understanding of it. A nonessential appositive is usually located in the middle of a sentence and is separated by commas, like this: “The chef, who seemed to be at the end of his rope, angrily took matters into his own hands.” Now see how easy it is to spot the essential appositive in this sentence: “The angry chef who tainted the food was immediately fired.” The essential words here are “who tainted the food,” since without this phrase the chef in question remains unidentified.