Parables are stories or images which teach some moral or truth. Generally parables are drawn from the everyday lives of the audience. Steinbeck wants the reader to regard the story in The Pearl as a parable – a story which tells a moral truth. He also alludes to a specific parable used by Jesus in Matthew 13:45-46, and it is insightful to compare Steinbeck’s story to this well-known parable about a merchant who gave everything he had in exchange for one pearl. In Steinbeck’s story, Kino will find a pearl but will give up certain things because of the pearl.
Steinbeck uses many Biblical allusions not only in his stories but also in their titles – e.g., The Grapes of Wrath, and East of Eden. So it is not out of character for Steinbeck to allude to a parable of Jesus in the preface of The Pearl. Indeed, he would have expected his reader to understand the meaning of parable and to be familiar with the specific parable which undergirds his story.