Depth of field is how much you want of something you want to have in your shot. “Depth of field is controlled by changing the aperture setting on your camera.” Any camera has a lens, an iris inside that can open and close to let more or less light in. “The larger the aperture, more shallower your picture will come out. The smaller the aperture the deeper depth of field will be.”
As the aperture size gets smaller the picture reduces the sharpness. Depth of field is the most important tools in your creative arsenal. Practice and experiment are the best ways to learn depth of field. As you shoot you can see the difference between a good depth of field and a shallow depth of field.