Aperture is ‘the opening in a lens’. When you hit the shutter release button of your camera a hole opens up that allows your camera's 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.
A lens with a maximum aperture anything wider than f/2.8 is referred to a 'fast', while others are referred to as 'slow'. This is because they don't let in as much light, so exposures require a slower shutter speed. If you want to blur the background, you'll want to use a fast aperture like f/1.8 or f/2.8, while for images that appear sharp from the front to the back of the frame you'll want to use a small aperture such as f/11 or f/16.
On the 4th and 5th row of photos, it shows me using an aperture of f5. This makes your photos have a lot of light. On the rest of the photos, the aperture was on f11 to give it the right amount of light.