This you tube clip is a great one for explaining aperture shots.
What is Aperture?
Put most simply – 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.
Aperture is measured in ‘f-stops’.