When an author uses something that stands for or represents something else, that something becomes a(n)
plot
character
idea
symbol

When an author uses something that stands for or represents something else, this becomes a symbol. This concept is known as symbolism. It is a technique used when things should not be taken literally. A symbol can be an object, event, action, person or situation that usually has a deeper meaning. The purpose for the use of symbolism is to enhance the writing and give insight into the story to the reader. The use of symbolism can also add a deeper layer to the story.

He went to walmart to get diapers