Aperture is ‘the opening in the lens.’
When you hit the shutter release button of your camera a hole opens up that allows your cameras image sensor to catch a glimpse of the scene you’re wanting to capture. The aperture that you set impacts the size of that hole. The larger the hole the more light that gets in – the smaller the hole the less light.
To get a shallow depth of field we use a large aperture- f 2.8
To get a wide depth of field we use a small aperture- f 22
Ex 1. If it is sunny out, you will use a low ISO.
Ex 2. If it is dark out, you will use a high ISO.
Ex 3. If you are in a gym with moving athletes, you will use a high ISO.