By changing your aperture settings (nothing else) it allows you to do the following;
a) change the amount of light. The higher number makes the aperture smaller, decreasing the amount of light (f.22) and the lower number (f.1.8) opens the aperture wider, allowing more light into the camera.
b) alter the depth of field. A bigger number (f.22) makes the whole scene of the photo in focus, whilst a smaller aperture (f.1.8) will put the subject in focus and make the background blurry.
