The aperture helps the photographer decide what they want to be in focus. It also links with the depth of field. For example the wider the aperture the smaller the depth of field. This means that the foreground is in focus and the background is blurry.

The F stop is what is used to measure the aperture

The image below has an aperture of 22. This means that the lens gets smaller and everything becomes in focus.

 

This image has an Aperture of 5 so the plane is in focus and the pencils in front and the toys behind. The lens expands in comparison to 22. This means there is a Medium level of: depth of view.