How to edit site and see changes locally after editing

Clone repo and edit it using any text editor. Install Jekyll (in Ubuntu you can just type apt install jekyll) and run jekyll serve in repo directory.

I don't understand how it works

GitHub supports Jekyll sites out of the box. To understand file organization, read Jekyll documentation. You don't need to understand Jekyll to contribute to the site, but you need to understand HTML and Markdown. You can just edit existing content or create new content using existing content as a template. For example, you can create a new post by copying existing post .md file in _posts and making appropriate changes. You can create a new project by copying existing .md file in projects and adding a link to it in projects/index.html. You can also try to use prose for editing, buy it's not recommended.