Hacktoberfest is a month long event that encourages participation in open source software projects through contributions like code, documentation, translation, and outreach. Participants who make four approved pull requests to public GitHub repositories between October 1-31 will earn a limited edition T-shirt. The document provides instructions on how to register and participate, defines key open source concepts like Git and GitHub, and outlines useful Git commands for initializing a repository, cloning a remote repository, staging and committing changes, and pushing commits to remote repositories.