This section provides examples of photos with various aperture settings. You can learn about aperture in the lessons section of the site.
For now, all you need to know is that the aperture controls which parts of the photo are in focus. A wide aperture provides a shallow depth of focus, meaning one part of the photo will be in focus, while the rest will be out of focus. A small aperture will often make the entire frame be in focus.
Check out the photos below, and read the descriptions to get an idea of what effect aperture settings have on photos.