The commenting feature allows users to comment on the activity of others. When a user comments on another users note, it creates a thread for the conversation to continue. 

Comment Threads - you will see the option to comment on any note that appears in the Note tab or Activity Feed or a specific lead's activity feed. Once you add your comment it will post underneath the original note or any other existing comments creating a thread. 

Comment Notifications - you will receive an in-platform notification in the upper right hand corner (bell icon) telling you if someone commented on your note or a note that you commented on. You will receive email notifications if you have enabled them under Settings > Account > Preferences.