Depth - Definition

Depth, or Depth Of Field, refers to the zone of the photo which is sharp and in focus and how big this area is. If the area that is in focus is large, we refer to it as a large depth of field. A large depth of field will usually mean most or all of the photo is in focus. On the other hand, if the area that is in focus is small and the rest of the image is defocussed, we refer to it as a narrow or shallow depth of field.
Here is an example of an image with a wide depth of field.

As you can see, most of the picture is sharp and in focus. Now here is an example of an image with a narrow depth of field.

As you can see, only a small area on the chain is in focus, and the rest is defocussed.