Aperture and Depth of Field
The pictures below each of the F-stop numbers represent the aperture that was used for that picture. The low F-stops have a shallow depth of field and the items in the background are blurry and as you increase the F-stop number the depth of field increases and the items in the background start to become clearer and clearer.
F-stop 3.5 F-stop 11 F-stop 22