@mentions in comments in SharePoint Online is a new Office 365 feature.
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.
Currently, this feature is available in Word and PowerPoint, and is coming soon to the new comment experience in Excel (available only to Office Insiders for now).
Microsoft have begun rolling this out to Targeted Release organizations, and are now continuing the roll out to all SharePoint Online organizations. During the course of the rollout, some Targeted Release organizations may experience sporadic access to this feature, and some documents and presentations will include this functionality, and others will not. Once the rollout completes, all Word and PowerPoint files will have this functionality, and all users will have access to the feature.
After rollout completion, users will receive a notice within the comments pane informing them of the @mentions feature.