Aperture is a camera setting that is related to the sharpness of the resulting photo. The setting can either blur out the background of a photo, focusing on something in the foreground, or bring everything in focus. This is determined by the size of the Aperture, which is the entry point of light in your camera lens. It thus limits the amount of light that enters the camera.
The size of the Aperture, or how far open or closed it is, determines how much light travels into the camera. This is expressed using f-stops such as f/4.5. The smaller the f-stop, the larger the Aperture. Thus, f/3.9 would be bigger than f/8. A large Aperture (and thus small f-stop) creates an image with a blurred background, while a small Aperture (and thus a big f-stop) brings everything in the picture n focus.