The aperture refers to the size of the iris (hole) within the lens. The bigger the hole, the more light gets into the camera. Aperture is measured in F-stops. A low number like F1.8 or F2.1 means a wider aperture. F16 would be a smaller aperture. A wider aperture (a low F-stop) means less of the image is in focus. It also means a faster shutter speed can be used.
Aperture - A large f-number such as f/32, (which means a smaller aperture) will bring all foreground and background objects in focus, while a small f-number such as f/1.4 will isolate the foreground from the background by making the foreground objects sharp and the background blurry.