When the aperture control ring of a lens is adjusted it mechanically clicks into a detent, causing it to stop precisely at the next setting (this is also true of most shutter speed and film speed dials).
The term stop is therefore used to represent a unit of exposure change in aperture, shutter speed, or film speed.
If any of these three settings is adjusted one stop up, the exposure of a photograph is doubled. Adjusted one stop down, the exposure is halved. Two stops up means four times the amount of light (doubled twice) and three stops down means 1/8th the amount of light (halved three times).