Depth of field is the amount of subject matter in front of and behind your plane of focus that appears to be in focus. When we say "that appears to be in focus" or "is acceptably sharp", understand that this is a continuum, it's not like objects in the depth of field range are razor sharp and then suddenly the sharpness falls off a cliff...
Change aperture to change your depth of field. You increase or decrease the depth of field in your photographs by changing the aperture setting on your camera. Aperture changes are measured in f-stops.
