A family of Microsoft word processing software products for creating web, email, and print documents.
You have text boundaries displayed (in this case it is showing the page margin). Many users find this display very helpful (for example, it can signal the existence of an empty and borderless text box, frame, or table that is causing formatting problems), but if you don't want to see it, go to File | Options | Advanced: Show document content and clear the check box for "Show text boundaries."