Answer:
ISO controls the sensitivity to light of a particular film or camera.
ISO is a measure of film "speed" or sensor sensitivity. ISO stands for International Organization for Standardization and replaced what used to be called ASA, which was American Standards Association. Both mean the same thing as far as photography is concerned.
Most folks today seem to be using digital cameras, which have the luxury of being able to change their ISO at will. Some cameras do it for you. The default ISO of a camera might be ISO 100, which would be a medium "speed" of "film." But in low light, the camera might crank the ISO up to 200 or 400 or higher, depending on the camera. Every time the ISO doubles, it's the equivalent of one full f/stop or shutter speed. Many cameras allow you to manually set the ISO higher, or lower, than the default, but be aware that the higher you put the ISO, the more likely you are to see "noise" in the image, which is the digital equivalent of "grain" in film. I never worry too much about it, as "grain: or "noise" sometimes is the price of a sharp image in lower than ideal light, but if you like things wire sharp and grainless, keep your ISO down and if necessary use a tripod.
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What the "ISO" knob on your camera actually does is recalibrate the meter.
Let's pull a number out of the air and say an "ISO 100" film requires 100 units of light to reproduce a medium gray (one that reflects 18 percent of the light that hits it--this is the "standard gray" all meters are calibrated to) as medium gray. And further, we'll say that if you set your shutter to be open for 1/250 second and your lens' aperture to be f/11 on a nice day, 100 units of light will fall on the film. There's a reciprocal relationship between shutter and aperture; if you change the shutter speed to 1/125 you will have to close the lens to f/16 to get 100 units of light; opening the lens to f/8 will require you to speed the shutter to 1/500.
If we change to a different speed of film, we need more or less light--an ISO 50 film needs twice as much light as an ISO 100 film; an ISO 400 film needs one-quarter the light. By changing the sensitivity of the meter through recalibration with the ISO knob, the meter will give good exposures on whatever film you use.