Aperture (F-Stop) – Aperture is determined by the camera lens. It functions like the iris of the eye, controlling the amount of light that is able to enter and make contact with the sensor. The Iris affects both light level & depth of field.

Wider Apertures let in more light and have a shallower depth of field. This will give you lots of bokeh (blurred background) & circles of confusion, allow for deeper focus pulling, but can also make it harder to focus on moving subjects.

Narrower Apertures let in less light and have a deeper depth of field. This will bring more of the scene into focus, doesn't facilitate much in the way of focus pulling, and is much easier to focus on subjects throughout the scene, particularly where multiple subjects are concerned. It will also create starburst effects on light sources. 

The effect of aperture can be seen more acutely when objects are closer to the lens, and the depth of field increases as you move your subject and focus further from the lens. 

Apertures can be fixed or variable depending on the lens. For some zoom lenses, the widest possible aperture will vary depending on the focal length.

In the majority of smartphones, the aperture is open at the widest point in order to let in the most light and cannot be adjusted. 

APERTURE - Depth of Field