Attaching your comments to specific parts of a document makes your feedback more clear. If someone else is commenting on the document, replying to their comments lets you have a discussion, even when you're not all in the document at the same time.
Select the content you want to comment on.
Go to Review > New Comment, and make your comment. If you want to make changes to any of your comments, just go back and edit them.
To reply to a comment, go to the comment, and select Reply.
Delete comments
Right-click the comment, and choose Delete Comment.
To delete all the comments in the document, go to the Review tab, click the down-arrow on Delete, and choose Delete All Comments in Document.
Source: Microsoft