F stops set the aperture in our lenses which determines how much light we let into the camera.
This results in differing depth of field effects in an image – how much is sharp and how much is out of focus.
The lowest number lets the most amount on light in and vice versa.
Confusing? I won’t bore you with the maths so here’s cake instead….
Cake Photo by Henry Be on Unsplash
If there are five of you, you get a bigger slice of cake then if there were 16 of you!
Your lens may have a maximum and minimum F stop capability which will change as you zoom in and out. Have a look on the rim of the front of the lens to find out.

Which F stops to use when:
Here I’m going to borrow Bryan Peterson’s grouping of F stops – it’s the best way I’ve found of explaining how to think of them – check out his excellent book Understanding Exposure for more info.
The Storytellers:

Usually anything from F16 and above.
Good for landscapes, large group photographs and where sharpness across the scene or subject is important.
Reykjavik : F16
Who Cares:

The mid range apertures F8 – 14 which typically create sharp and crisp images
Good for any type of photograph and good as a default setting.
Cambridge: F11
The Isolators:

Typically anything below F8
Good for portraits, macro, flower and wildlife photography.
Enables backgrounds or distractions to be blurred out placing emphasis on your subject.
Botanical Gardens, Cambridge: F5.6
Shooting with a smartphone
Use portrait mode to get a shallow depth of field effect in your photos. If you can’t get the effect you want then use the lens blur tool in Snapseed or Lightroom mobile. If shooting portrait on an iPhone, you can change the f stop afterwards in the Apple photo editor.