Every piece of content on your site can be reviewed/commented on by the user. By default, they must be registered and logged into an account in order to leave a comment, but this can be changed in the cpanel if required.
Each comment must include a title, which shows as part of the comment once approved.
Comments can be plain text only, and will show in a section directly underneath the comment itself.
This button saves the comment in the system, with the approval appearing for the website owner under the "To Do List" section of their cpanel dashboard (below). If the user is not logged in, they'll be redirected to the login page before being able to post the comment.