Within a lens you have aperture blades which control how much light gets into the lens and reaches the sensor, this is called the aperture. This setting called the aperture and is controlled with an F/number. The larger the F/number the more the aperture blades are going to be closed, therefore less light will reach the sensor and on the other hand the smaller the F/number the more the aperture blades will be open so more light will reach the sensor. For example F/8 is going to let in much more light that F/22 This aperture setting is essential to the exposure.
The aperture you use to take an image also affects how much of the photo is in focus, also known as the depth of field. The depth of field can be useful to change depending on the desired effect. During a landscape you might need a deep depth of field to keep everything in focus however portraits might want only a shallow depth of field to blur out the background. The blurred background is known as ‘bokeh.’ The quality of the bokeh can be very soft depending on the lens you use. The lower F/number allows you to have a shallow depth of field, resulting in more of the photo being out of focus and higher F/numbers give a deep depth of field making most of the photo in focus. The two photos in this article use a deep depth of field. This photo uses a shallow depth of field, leaving only some of the track in focus.
When you buy a lens the smallest F/number will be written on the front. The average 18-55mm kit lens is 4-5.6. There is a range of smallest apertures on most zoom lenses because as you zoom the smallest F/number possible changes. With prime lenses you will have one smallest aperture. For example Nikons 50mm is available in F/1.8 or F/1.4 models, the 1.4 being the more expensive options.