What is the "medium"? In photography, it's the part of the camera that is actually capturing the image. On older 35mm film cameras, it was the film. On todays digital cameras, it's the digital sensor. So what does the medium have to do with light control? Well, lets step back in time a bit.
In the days of film photography, film came in different "speeds". These speeds were referred to by ASA number. With digital cameras we use ISO numbers. The lower the ISO number, the less sensitive the film is to light, the more light it needs to get a good exposure. The higher the ISO number, the more sensitive the film is to light, the less light it needs for a good exposure. The purpose behind the different speeds is to allow photographers to shoot in different light conditions. If a photojournalist is going to be shooting a night event or indoor event with little light, he would shoot a higher ISO to be able to get faster shutter speeds.
But, as it seems with everything in photography, ISO had a side effect. Photographic film is actually created by mixing small solid particles of crystals (typically silver halide) into a gel mixture and coating a plastic film with the mixture. When you expose the film with the mixture on it to light, it records an image on the film. A small ISO number means the particles are very small but not as sensitive to light. The smaller particles mean a finer grain to the film and therefor, good detail is recorded. Larger particles mean better sensitivity to light but also larger grain, so you could lose some detail in the image and you would actually see the film grain in the final image.
Oddly enough, the same effect carries over to digital sensors but not for the same reason. A digital sensor consists of millions of tiny pixels, or picture elements. Each pixel is an electronic device that reads an analog signal (in this case, light) and coverts it to a digital value. As is true of all electronic sensors, there is always some background noise that it picks up. Think back to the days when you had televisions that got their signal off antennas. A nice strong signal produced a good picture. There was background noise there but the station signal was strong enough to override that background noise. As the signal got weaker from the TV station, the background noise would "bleed" through and show up as what we called "snow" on the TV screen. The same is true of a weak radio station where static starts to be heard. That static is the background noise.
The pixels of a digital camera sensor work much the same way. Power applied to each pixel renders a certain amount of background noise. But a good strong light signal would overpower the background noise and give you a usable light value for that pixel. This would represent a low ISO value such as 100 ISO. Increasing the ISO value on a digital camera actually just increases the amount of power to the pixels so they become more sensitive to the light signal. The side effect (yes, yet another side effect) is that the greater amount of power increases the background noise. On images that are very dark or have large, dark, uniform areas, the background noise can bleed through and creates "snow" or spots on the final image. We usually use software yo try to clean up as much as possible but often, a high ISO can render an image that looks grainy, like old film images.
On newer digital SLRs, the sensors and image processor chips have gotten quite good at producing cleaner images allowing you to get very usable images even at very high ISO values. So ISO can be used to take images in low light conditions so that you can use higher shutter speeds.