Depth of field is the amount of space in your images that is before and beyond the point of focus. The depth of field lets you determine if your subject and the background can be in sharp focus together. Though a camera can only technically focus on a single point in space, the depth of field determines how much of the picture is within an “acceptable focus” for the human eye.
The desired depth of field differs depending on the type of photography you are doing. For example, with a landscape you will likely want a large depth of field so that most of the scene appears in focus, whereas with a portrait, a smaller depth of field keeps the model as the focal point and lessens any distractions in the background.