This refers to the difference in the amount of light transmitted through the lens as the  lens aperture  is moved from one setting to the next.  This difference in the amount of light from one stop to the next,  is either twice or half as much light depending upon the way the aperture is moved. 

For example, twice as much light reaches the film, or CCD, if the aperture is moved from f11 to f8.  As mentioned, the difference between settings is one stop.

And conversely half as much light reaches the film when the aperture is moved from f 8 to f11.
 

Open or wide apertures

stopped down or closed apertures

f  stop  setting	f2	f l.4	f2	f2.8	f4	f5.6	f8	f11	f16	f22	f32	f45	f64	f90
units of  light transmitted	8192	4096	2048	1024	512	256	128	64	32	16	8	4	2	1

Notice how the units of light doubles from one aperture to another as we move from f90 to f2.

How does aperture affect depth of field?
How are the f stop numbers worked out?
What is the difference between each f stop setting?