When you comment on a document or presentation and use the @-sign with someone's name, the person you mention receives mail with a link to your comment. Clicking the link brings them into the document and into the conversation.

Now you can use @mentions in Word and PowerPoint to draw someone’s attention to comments in a document or presentation you’re collaborating on. Just add the @ sign, then type the name of and the person in your organization you are collaborating with.  That person receives an email with a link directly to your comment. Clicking the link brings them into the document or presentation and into the conversation. Using @mentions in Office can help take the lag time out of the collaboration process helping you get more work done, faster.

Currently, this feature is available in Word and PowerPoint, coming soon to the new comment experience in Excel (available only to Office Insiders for now).