So depth of field is the area of sharpness within a photo that will appear in focus. The lower the aperture
f-stop the smaller the depth of field F-stops 1.2 1.4, 1.6, 1.8 can all be used to reduce the focal depth and create what is called BOKEH
(Click for more information).
A deep depth of field means that almost everything in your image will be in sharp focus, from things in the near foreground to things in the far background. Think of a basic phone camera, where everything in the image is sharp and in focus, even things that are far in the distance.
We use a narrow aperture to achieve a deep depth of field when photographing wide vistas, like landscapes and city skylines, where you want everything to be in focus.
A shallow depth of field (as shown in the image) means only the subject will be in focus and the rest of the image will be out of focus. We use a large aperture to achieve a shallow depth of field when photographing a portrait against a busy background, so the person will be in focus and the background will be blurred to not distract the viewer.
If the lens if fixed to the camera the depth of field is constant across the width of the image. We can distort this by shifting the lens (tilt shifting) or by using a rubber mount. See Creative Techniques