The aperture of a camera is a small hole in the lens which is often referred to as the diaphragm. The aperture dictates the amount of light that can pass through the camera. With a larger hole more light can get through. With a smaller hole less light can get through. The aperture never closes and is always open. The aperture is also found on the lens of the camera.
The aperture is represented as f-stops and is abbreviated such as f/5.6, f/11, f/22, etc… The easiest way to remember the aperture values is to remember that the smaller the number the bigger the hole. A bigger number results in a smaller hole. So when we say a large aperture we are talking about a large hole and a low number (f/5.6 for example). When we say a small aperture we are talking about a small hole and a high number (f/22 for example).
The image below is an example of the diaphragm of a lens. The aperture value is what dictates how big the hole in the middle is.
When a large aperture is used (f/5.6 for example) a lot of light is able to pass through. This results in a quick burn of your image and a smaller depth of field. The smaller depth of field is a result of the camera not having enough time to get enough light information for the different parts of your image. These additional parts turn out blurry and the result is out of focus.
When a small aperture is used (f/22 for example) less light information is gathered which results in a slower and more balanced burn of your image. With a smaller aperture you have a greater depth of field allowing close and far away objects to appear in focus.