When you comment in a file and use the @-sign with someone's name, they receive an email with a link to your comment. Clicking the link brings them into the document and conversation.
Select Review > New Comment.
Type @ and a name, then pick the name you want. That person will get an email that guides them to the comment in the file.
When someone mentions you in a comment, click on the link in the email you receive, then add your response directly to the comment that needs your attention.