F/Stop

This pretty much sums it up. "My favorite analogy for exposure is filling a bucket with water. A bucket is of fixed size and needs a certain amount of water to fill it, just like the sensor in your camera, which is of a set sensitivity (the ISO) and needs a certain amount of light to optimally capture an image. To fill a bucket, you can pour a small stream of water for a long time or a fast stream of water for a short time. Either way, you end up with the same amount of water. In photography, the size of the stream of the water is analogous to the f/stop, the length of time you pour is analogous to the shutter speed, and the size of the bucket is analogous to the ISO. Broadly speaking, from the bucket's point of view, it doesn't matter which combination of stream size and length of time you choose as long as the right amount of water ends up coming in. Photography is the same; within limits, your camera is indifferent as to the combination of time and amount of light as long as the right amount of light eventually arrives."

F/Stop in a nutshell