Oops, my bad. A stop is equal to one half the light or twice the light of something.
For instance. 1 stop more is twice the light of something. 1 stop less is half.
This translates into changes in shutter speed, aperture and ISO. The most common change is shutter speed.
If you're correctly exposeing at 1/100 and the the light changes to be brighter by one stop, then the correct shutter speed is 1/200. Conversly, if the light loses a stop, then the correct shutter speed is 1/50.