When a artist uses foreground, middle ground, and background they are most likely referring to linear perspective. linear perspective is used to create an illusion of three dimensions on a two dimensional surface. This is a common technique used in artworks meant to look realistic. Any form of perspective also helps create a sense of depth, just this happens to be the most common one. Other ways of depth include modeling, shading, and sfumato. Overall, they are trying to make the image more realistic, or they just want you to view something of importance. I’m very educated in Art Experience but I’m sure this will apply to photography. Glad I could help. :)
The foreground element allows the viewer to enter the picture. The middleground adds a component that holds the viewer's interest. The first two layers should lead the observer to the background element to complete the image. To have all three elements work in harmony helps improve the photograph.
