What is Aperture? Aperture is a device... ...that's all? Haha, just kidding. Of course it is a device, it control the light goes into the camera.
Some Aperture might not look exactly like this, but the theory and the way Aperture open and close are same, for sure.
There are several option in setting the size of Aperture, and we called it a "stop". The number "f/2", "f/2.8", "f/4", "f/5.6", "f/8", "f/11" and "f/16" are the "stop" or someone called it a "f-number". Different entry level camera might have different "stop" as well. As you can see, the smaller the number, the larger the Aperture. It is mathematics, assume the "f" is 1, and the larger the denominator, the smaller the value. Easy :)
Do you remember the Black Box mentioned in "How does Camera work?". The hole on the wall opposite the film, it has the same usage as Aperture.
However, the larger the Aperture open, the more light goes in the camera. The smaller the Aperture open, the less light goes in the camera. Aperture is one of the factors that decide brightness of the picture. If you found your picture not bright enough, you should open the Aperture larger ( if there is any "stop" available).
Here's the picture that shows the differences between the stops.
If you found your Aperture already the largest but the picture still not bright enough... ...Well, we will discuss in next chapter :)
For SLR (Single Lens Reflex) camera, the Aperture can be larger or smaller because it depends on the Lens. Some special Lens have a stop at "f/1". But for most compact camera, I sorry to say that, you cannot have a larger or smaller Aperture since you cannot change your camera Lens.