What Is Depth of Field?
What is in-focus in your pictures? What is out of focus? Depth of Field (DOF) refers to the area of your picture that is in focus. The word depth is used because your lens’ aperture setting determines what measure of distance within your picture is in focus.
A picture can speak a thousand words. Which is why I like photography; I am able to conserve my words. The image above from Augustine's Notebook demonstrates the concept of DOF pretty well.