Stops are a doubling (or a halving). 2 seconds is one stop longer than 1 second, while 1/2 seconds is one stop shorter. 1/250th is one stop longer than 1/500th, which is one longer than 1/1000th, etc. Aperture is measured in the same way, in that each stop of aperture lets in double or half the amount of light, it's just not obvious because of the way the numbers read: 2.8, 4, 5.6, 8, 11, 16 are all one stop apart.