Focal length/Depth of field is the cameras capability of a lens to magnify the image of a distant subject. If the photographer has used a small depth of field the main focus of the image will be in focus and the rest of the image will be getting more and more out of focus, this style is normally used in portrait photography.

However if the photographer is shooting a landscape image they would normally use a small aperture so that the whole of the image will be in focus.

Large aperture = Small f-number = Shallow (small) depth of field

Small aperture = Larger f-number = Deeper (larger) depth of field

So for example this image was taken with a  large aperture setting so that the camera would be able to focus on one particular part of the mandala sheet,