Depth of field refers to distance between objects that are in focus. Narrow depth of field means that objects that are close together can be in or out of focus. Wide depth of field means that objects that are far apart all appear in focus.

This photo has a very narrow depth of field. Foreground and background are out of focus and so are the buttons next to the one that is in focus even though they are close to the button which is in focus.

This photo has a wide depth of field. Everyone in the photo is in focus, even though they are far apart.