Depth of field is the distance between the nearest and the furthest objects within a focused image. And how focused separate elements of the image are.

In these images the depth of field is alternative as in the first image the main focus (the branch) is in crisp focus however the further objects in the image are unfocused/blurred. But in the second the depth of field is slightly more balanced as the background although not in full focus is more visible.

The depth of field of an image can be altered through the use of the camera’s aperture (approx f/1.4 to f/22)

As you can see in this diagram a lower aperture (eg. f/1.4) causes the background of the image to be unfocused with the close/main subject remaining in focus. However a higher aperture (eg. f/22)  the whole image is focused.