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ISO is a measure of the sensitivity of the imaging surface, I.e., the film or digital sensor. A higher number means that it needs less light to get the same exposure.
So if you can take a shot at f/2.8 1/30th at ISO 100, you can take it at f/2.8 1/60th at ISO 200.
Going to higher ISOs has a cost, though--with film, higher ISO film is grainier; with digital, higher ISO settings on the sensor are noisier. Better sensors have less noise at higher ISO, so I shoot constantly at ISO400 on my digital SLR, but ISO400 on my little point & shoot camera was unusably noisy.
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