The Discussion tab serves to share information concerning a particular record among users. This tab is included in every record's details and you can always find it at the end of the individual tab panels.
Every user that can access a record can add new Comments in the Discussion.
If you want to add a new comment, type any text into the Comment section and confirm by clicking the green button Post comment.
You automatically become a discussion follower after posting your comment. Every user that is a discussion follower is notified about new comments in the discussion by notifications.
If you want to add other users as followers, click the Add a user button.