To insert a comment in a document, put the cursor in the text where there is a question or comment to be made.  Go to the Review Tab and then navigate to the Comments Group. Choose New Comment to insert a question, idea or clarifying statement. As the author resolves the questions/comments, the comments can be deleted by right clicking the comment and choosing to delete the comment.

(For a shortcut, you can also right click next to the text and select New Comment)