Comments allow you to add thoughts on a record outside of the form itself, and also allow you to @-comment people in your workspace to bring them into the conversation.
Commenting on Activities
When viewing a record, you can utilize the Activities panel to add comments to the record and to @mention people in your workspace.
Use the green 'Add Comment' to add a new comment.
You can upload files as well.
Commenting on Tasks
When viewing a specific record, you can add comments to any tasks pertaining to that record. To do so, navigate to the task panel of that record.
Click on the task name to open up more details about it.
You can add a comment using the 'Add a comment...' field.
You can also comment on existing events for a particular record.
Access the event you wish to comment on by clicking it from the record's event panel.
Add the comment using the "Add a comment..." field