The Question and Answer section for The Secret Life of Walter Mitty is a great resource to ask questions, find answers, and discuss the novel.
One of the main themes in the story is the compensatory function of the imagination. Walter Mitty has a "secret life" not simply because he is an imaginative fellow, but because he is an unhappy man. Though we begin the story in his imagination...
• passivity
• daydreaming
• forgetfulness
• bad driving
Generally each dream represents some kind of Frustrated/Repressed Desire that Walter has. Check out Frustrated/Repressed Desire below:
https://www.gradesaver.com/the-secret-life-of-walter-mitty/study-guide/themes