The best way I have ever heard to explain ISO is to imagine that your camera is a bee hive and that ISO represents the number of bees and light is represented as honey.
When there are just 100 bees they can collect x amount of honey in a certain amount of time. If you double the number of bees to 200 hundred (equivalent to increasing by 1 stop) they could collect the same amount of honey in half the time or double the amount of honey in the same time. Now if you have 400 bees they could collect double the amount that 200 bees could in half the time or collect double the amount of honey in the same time and so on and so on.
So for every stop increase in ISO your sensor is able to gather light twice as fast.
Here are the common stops for ISO:
50, 100, 200, 400, 800, 1600, 3200, 6400, 12500
Increasing your ISO doesn’t come for free. Everytime you increase your ISO your images will become more noisy, which means they will contain little bits of what almost looks like graininess. This is due to the way that increasing ISO works. Basically when you increase your ISO you are increasing the voltage running through your sensor, this in turn results in electrical interference in your images in the form of noise.
Camera manufacturers have made great improvements in this area in recent years but it is still better to use a lower ISO if possible to achieve the best image quality.
