An f-stop, f/stop or f-number corresponds to the lens’ ability to gather light, and is the numbering system representing the aperture.  (Some camera manuals actually refer to the f-stop as “aperture value”). Any lens set at f/8 will admit the same about of lights as any other lens set at f/8.

The series is:

1    1.4    2    2.8    4    5.6    8    11   16    22   32

Aperture Diagram

Although the number system can be confusing, it may be easier to understand if you remember these two things:

1.)    Larger numbers mean less light (smaller apertures)

2.)    Although the f-numbers do not double from step-to-step, the amount of light does

Why is it important to know and understand these numbers? One reason is so you can adjust other settings accordingly, such as shutter speed. For example, f/2.8 admits twice as much light as f/4.  So if you were shooting at f/2.8 and switch to f/4 but want to keep the same exposure, you will need to make shutter speed two times slower.

How does it affect image quality?

As the f-number increases, so does depth of field (see photo below).  So a photo taken with a high f-number, such as f/32 will have more of the objects in the picture in focus (top left half of photo).  A similar picture taken with a low f-number, such as f/5 will tend to have one object in focus, and the rest out of focus (bottom right half of photo).  Playing with this setting can give you interesting shots, especially when used for nature photos.

f-stop example

Here is a fun video to help you remember how the f-stop system works.

For a more detailed explanation of f-stops, along with the math behind the number systems, read this about the importance of understanding f-stops or math behind them.