When you setup GitHub pages, by default your content is available at https://<GITHUB_USERNAMNE>.github.io/<REPO_NAME>/.
However, if you create a repo with the exact name <GITHUB_USERNAME>.github.io, GitHub will automatically turn GitHub Pages on, and make this content avialable at https://<GITHUB_USERNAME>.github.io/. Kind of like your GitHub home page!
For example, I set up a sample at:
https://github.com/codeadamca/codeadamca.github.io
The repo is avilable as a website at: