Depth of field is how much the image is in focus or the distance between the nearest and furthest objects in a photo. Depth of field is controlled by the aperture, the smaller the aperture the narrower the depth of field. if the depth of filed is narrow the only the closest image to the lens will be in focus although if you have a larger depth of field and a larger aperture everything will be in focus.
Below are some pictures I took using depth of field to try and get the object in front of the lens in focus and the rest out and to try and get everything in focus such as the object and background.