Aperture controls the amount of light reaching the image sensor and is usually expressed as an "f-number": f/1.4, f/2, f/3.5, f/5.6, f/8, f/11, f/16, f/22, f/32.
Changing the f-number changes the size of the opening through which light enters the camera (the aperture) and thereby changes the amount of light passing through the lens. High f-numbers stop the aperture down, allowing less light into the camera, while low f-numbers increase the size of the aperture and let more light into the camera. Going from an aperture of f/4 to f/5.6 reduces the amount of light entering the camera by half and halves the brightness of the image on the image sensor.
Aperture also controls the depth of the area in front of and behind the subject that appears to be in focus. The higher the f-number, the greater the depth that appears to be in focus. In contrast, lowering the f-number reduces the distance in front of and behind the subject that appears to be in focus. This is referred to as "depth of field".