Aperture is the amount the lens of the camera opens. This one is a little confusing. The smaller the number, the wider the opening. Aperture is measured in F-stops (i.e. f-1.8, f-4.5, f-22). The lower the f-stop, the shallower the depth of field (DOF), which creates a more blurry background and a clear foreground.

Aperture determine how much light is allowed into the camera. The shutter speed works in sync with the aperture as well as the ISO to capture a more shallow DOF or wider DOF