Aperture
Aperture is the size of the whole within the lens. The bigger the whole is the more light that will be let in the camera, the smaller the whole is the less light that will be let in the camera. Aperture is measured in F-stops, a number like F1.8 or F2.1 are examples of wide aperture so more light will be let in. F16 would be an example of smaller aperture so the less light light will be let in. The wider the aperture the more light will be let in sop this means that the image will be less in focus, also this means a faster shutter speed could be used.