Aperture is a setting which is a little bit like ISO, the difference is that the backgrounds focus changes throughout the picture but in ISO the background doesn't go blurry. Aperture also changes the amount of light in the picture shown below in the gallery.
Why is aperture important ?
Aperture is important because if you want a picture to look professional then you might need to change some setting such as the aperture. If you are taking a picture of someone and they are smiling happily then you would like a bright picture, But then if you are taking a sad picture then you might need to make it darker.
This is the best picture out of the gallery because the actual flower is in focus and the background is out of focus which makes the flower stand out in the picture. Also the lighting in this photo is also is right and not to over exposed.
This picture could be improved by lowering the f-stop which means that the lens hole will go smaller allowing small amounts of light to pass through which will then make the picture less over exposed and easier to see.