Placeholder text
Alternatively referred to as dummy text, placeholder text is text that reserves a specific place on a webpage or other document for images, text, or other object. When being replaced, the placeholder text is removed and the actual text, image or other object is put in its place. Placeholder text is often used in document templates, such as a newsletter template where you would replace the placeholder text with your actual newsletter content.
A placeholder can also be used for when a value or variable is not known at the time, and a placeholder is used as a temporary solution until a proper value or variable can be assigned.