Depth of field is how much of your picture is in focus. If you blur the background of your picture, it will bring more attention to the subject. Depth of field can be controlled by the aperture of your camera. The aperture is the hole that your camera takes the picture with. it can be big or small. The bigger the aperture, the less in-focus the pic will be.
https://www.digitalcameraworld.com/tutorials/cheat-sheet-wide-vs-narrow-aperture
Aperture is an important way we can determine how an image turns out. Aperture is how much light gets through your lens. When you have the subject focused in on and everything, that is having a shallow depth of field. The bigger the aperture gets, the faster the shutter speed should be. A portrait would be a good place to use a shallow depth of field.