Hashim Warren introduces Challenge 13 of the #100DaysOfGatsby, which focuses on adding user comments to a Gatsby blog. This challenge is part of a broader initiative encouraging both beginners and experienced users to enhance their skills in using Gatsby, a popular static site generator. The challenge provides three methods for incorporating comments: using JavaScript-based services like Disqus and Facebook Comments, utilizing server-side rendered comment services for Markdown-based pages, and developing a custom commenting system for greater control. Each method has its own pros and cons regarding performance, privacy, and complexity. Warren also suggests engaging with the Gatsby community on platforms like Discord and Reddit for assistance.