The Publish instance is used to make the designed application available to the public. Whatever the content developed in Author, if the author publishes the content then the content will be available in publisher. Basically, If the content is presented in the publisher then the content will be shown in live else it will through 404 Page Not found error. For more information on AEM , go through the below syllabus. AEM Author Concepts
AEM Author Tutorial, which will give overview of content authoring. This helps from beginners to professionals