ISO is the abbreviation for a rating scale that indicates how sensitive the camera film or sensor is to light and is equivalent to the old film ASA scale
the lower the value, the LESS “sensitive” to light (ie. less amplification of the sensor signal) and thus you need a larger aperture, longer shutter or stronger light or a combination of these to get the same exposure if you lower the ISO value.
in general, photographers aim to use the lowest ISO setting possible for the conditions, subject and lens as the lower the ISO the better the image quality in terms of image noise and dynamic range
in low light conditions or with fast moving subjects, one may need to increase ISO to reduce camera shake or subject motion.
most cameras have a starting or “base” ISO of 100 or 200 although some newer cameras have a “dual base ISO” although there is still no change to the sensor's signal to noise ratio or “sensitivity”, these cameras just apply the amplification earlier in the signal chain to provide a cleaner output.
each time the ISO is doubled, this equates to 1 stop brighter (ie. it is twice as bright)
image noise increases (and dynamic range and image quality decreases) as ISO increases and the extent and type of this image noise depends upon the photosite size on the sensor and the technology used
most photographers using flash or tripods will prefer to use the base ISO for the majority of their shots
most hand held photography is done at ISO 100-400 which allows a fast enough shutter speed to minimise camera shake in most conditions outdoor in the daytime
indoor hand held photography without flash often requires ISO 800 with a fast lens such as f/2.0
sports photography with fast shutter speeds of 1/500th second or faster often require ISO 1600 or higher.