Depth of field is the zone of acceptable sharpness within a photo that will appear in focus. In every picture there is a certain area of your image in front of, and behind the subject that will appear in focus.
This zone will differ from photo to photo. Some images may have very small zones of focus which is called shallow depth of field.
Others may have a very large zone of focus which is called deep depth of field.
3 main factors that will affect how you control the depth of field of your images are:
distance from the subject to the camera, and
focal length of the lens on your camera.