f-stop (or f-number) is a measurement of the size of the aperture (opening) of the camera iris. If the aperture is wider, more light enters the lens and the picture is brighter.
F-stops can be a little confusing at first as the numbers do not behave as you would expect. Higher f-stop numbers indicate a smaller aperture diameter - in other words, as the iris opening decreases in size the f-stop number increases.
F-stop settings are normally written with a forward slash like so: f/16. Common f-stops are:
f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, and f/22.
Note that a small f-stop (large aperture) results in a small depth of field and a larger f-stop (smaller aperture) gives a large depth of field.