When you comment on a document, presentation, or spreadsheet 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.
— Read on support.office.com/en-us/article/use-mention-in-comments-to-tag-someone-for-feedback-644bf689-31a0-4977-a4fb-afe01820c1fd