Every photograph is an exposure, created through the use of three controllable factors: Aperture, Shutter Speed and ISO. Changing one of these settings means changing the other settings. Each element has a trade off that effects the image.
Exposure is measured in ‘stops’ each stop is half or double the amount of light of the previous number. For example f/4.0 is half as much light as f/2.8, 1/125th second is twice as much light as 1/250th second, ISO 400 is four times as much light as ISO 100. – on a digital cameras this is broken down further into thirds of a stop, each click on the camera control dials changes the exposure by 1/3 of a stop.