You can write a Comment in a Thread, and people can reply, react or post their own comment to continue the discussion.
When you have written your text as a Comment in the thread, use Post Comment to instantly publish the comment. Your comment will be visible to anyone who has permission to see the thread. If you want to change anything you can edit the comment.
You can notify people when you write a comment.
You can reply to someone's comment. Your reply is published in the thread and an email is sent to the author of the comment.
Reacting to a comment is a great way to encourage and acknowledge a comment by letting the comment author know how you feel. It is a quick and light-weight way to participate that does not require any text, nor send an email.