In photography, the term “stop” is used to denote a relative measure of exposure. In other words, a stop is a specific amount of light.
It may seem obvious but aperture, shutter speed and sensitivity (ISO) are all interrelated. You can adjust each individually to alter the amount of light hitting the imaging plane. This is known as reciprocity. To understand, and discuss, reciprocity and the interplay between aperture, shutter speed and ISO it’s easiest to talk in terms of “stops”. So, lets start with shutter speed.
Shutter speed is nice and simple to understand because it is measured in units which make sense to us; seconds or fractions of seconds (unlike aperture which, at first, seems rather odd in it’s numbering system).
Remember that a stop means exactly half (or exactly double) the amount of light. If your shutter speed is 1/500 (one five hundredth of a second) you can halve the amount of light getting through by opening the shutter for half the amount of time (1/1000). You can double the amount of light from 1/500 by holding the shutter open for twice as long (1/250) and so on.
1/500 - 1 stop = 1/1000
1/500 + 1 stop = 1/250