Depth is the apparent distance from front to back or near to far in an artwork. Techniques of perspective are used to create the illusion of depth in paintings or drawings.
When drawing or painting landscapes, you always want to work back to front, starting with the sky first. Many beginners want to get right in there and draw the subject first, but that’s not affective. By perfecting the background first, things can then be applied and overlapped. You never want to draw something in and then try to put the background in later, around it. It will show. In landscapes, things become lighter in color and less detailed as they go further away.
Landscape painting teach us about very principles of depth and distance, you can see how artist applied the paint in gradual horizontal layers, starting with the sky, and then working down into the many atmospheric layers.