The depth of field denotes the extent of the image that is in focus. A number of factors determine this, including the aperture that you are using, using hyperfocal distance, the sensor size, the distance between the camera and the subject and the subject and the background being the major factors that determine the depth of field of your images.
An image where the majority of the frame is in focus is said to have a large depth of field. Alternatively, an image where only the subject is in focus and the rest of the frame or a majority of the frame is out of focus, is said to have a shallow depth of field. Both shallow and large depth of field is required in photography.