What is aperture?
Aperture is the amount of light being admitted through the lens as you capture your subject.
How can you tell how much light that is?
In the example image, the aperture, also known as the f-stop, is the number 4.5 (F4.5).
Adjusting the aperture allows you to get creative. Think of a crisp, focused image (foreground and background), compared to a focused foreground subject but with a blurred-out background.
Using an aperture of f/4.5 compared to f/16 helps create the blurred background effect. Dialing in f/16 gives you more of the postcard type creation.
What is going on?