The depth of field refers to how much of the picture they choose to have in focus. Sometimes you want to have attention on a certain object and to do that you would reduce the depth of field in a scene to make the background blurry. You would use your aperture setting to change depth of field. A shallow depth of field means the background is blurry and the object in focus is very sharp. A deep depth of field is when the focus of the image can be all around the image because nothing is blurred.
There is a whole in the lens that lets in light and it is called the aperture. You can control the size by changing the scale of f-stops. The bigger your aperture, the more shallow your depth of field will be so you can have more focus on a subject. The smaller the f-stop number, the larger the aperture. You can change your aperture setting if your camera has an aperture priority mode or a manual mode. When taking landscape photos it is better to have a deep depth of field because you want the attention on everything, you would want a smaller aperture for this. Portraits usually use shallow depths of field so the person can be more in focus.