A stop is the doubling or halving of the amount of light entering the camera. It is so called because in years gone by the lens aperture was changed by dropping in a 'stop' with a different sized hole in it.
Adjustments can be made by altering any one (or all) of shutter speed, aperture or ISO.
Shutter speeds and ISO speeds are logically adjusted by doubling or halving the number whereas this is no so obvious with the aperture as the difference of a whole stop is the increase or decrease by a factor of 1.4.
The main aperture numbers for a stop follow the following progression:
f1.0, f1.4, f2.0, f2.8, f4.0, f5.6, f8.0, f11, f16, f22, f32
Numbers in between these above will be either 1/3rds or 1/2s of a stop depending on how your camera is set up.