– It determines the depth of sharpness area of the image, or depth of field!
Shallow or wide strips of sharpness are obtained by using different f-stops. Shallow depth of field (using smaller f-stop numbers) means that the area of sharpness is very small and all other objects including nearer or farther objects are blurred in the image. Obviously, larger sharpness area is obtained as depth of field becomes deeper (using larger f-stop numbers). So, you can capture an object in near and far distance as sharp as it is, and leave everything else blurry. In contrast, you can capture a landscape with everything sharp in it, from near to far distance.
Image story: A pleasant Saturday morning walk in Victoria park, Auckland city, New Zealand. End of winter and there was no leaves on trees. These beautiful seeds, like black dots in white lines of tree branches made me think of using a shallow depth of field to capture the beauty of a single dot in a shot! I set the aperture of my lens on its widest, f/4, and shoot a dot. The original background of the image was green and I lightly touched the Hue of background to make this nature paint more eye catching.