After you open a pull request in a repository, collaborators or team members can comment on the comparison of files between the two specified branches, or leave general comments on the project as a whole.
You can comment on a pull request's Conversation tab to leave general comments, questions, or props.
You can also comment on specific sections of a file on a pull request's Files changed tab. Adding line comments is a great way to discuss questions about implementation, or provide feedback to the author. You can reply directly to a line comment from the Files changed tab.
Adding line comments to a pull request
Under your repository name, click Pull requests.
In the list of pull requests, click the pull request where you'd like to leave line comments.
On the pull request, click Files changed.
In the comment window, type your comment. When you're done, click Comment on this line.
Anyone watching the pull request or repository will receive a notification of your comment.