Final answer:
A watermark is the feature that allows adding background text or images to a document; typically found in word processing software's 'Design' or 'Layout' menus.
Step-by-step explanation:
The feature that allows you to add text or a picture in the background of a document is commonly known as watermarking. A watermark can be a faded background image or text that appears behind the main content of the document, often used to identify the document's status, such as "draft" or "confidential", or to brand it with a company logo or copyright notice.
In many word processing applications, including Microsoft Word and certain online document editors, you can easily insert a watermark into your documents. You typically find this option in the "Design" or "Layout" tab within the document's menu. Besides watermarks, there are several cool features in digital textbooks and documents, like the ability to highlight, annotate, or use a table of contents for navigation.