When photographers mention "stop"; it means a measure of light, a stop represents a doubling of light. Any time there is a doubling of light (whether its about light in scene or light hitting the sensor) they say that light has increased by one stop and vice versa. Experienced photographers can recognize one or two stops of light by eye. (I cannt :-#)
[LEFT]1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000 and 1/4000; each shutter speed is double the previous one (almost; except the first one) i-e there is a one-stop difference in the amount of exposure...Similarly f1.4, f2, f2.8, f4, f5.6, f8, f11, f16 and f32*; each aperture is twice as big as the next one (aperture is circular - there is "pi" involved therefore its not mathematically double values like shutter) i-e difference of one stop of light exposure between each of these apertures.