Depth of field is when a specific subject is in focus. The background can either be in focus or not. When the background isn’t in focus that’s called a shallow depth of field. When the background is sharp or in focus that’s called good depth of field. You would change how close you are from your subject depending on how you want your background. If you want a shallow background you would get closer to the subject and have a bigger lens too. If you were to want a sharp background you would get further from the subject and a smaller lens. By changing your aperture would affect your background too. The smaller the aperture the deeper the depth of field. When shooting depth of field it’s best to have your camera mode on AV.