ISO is the level of sensitivity to light; either film sensitivity or digital camera sensor sensitivity.
In case you’re interested, ISO is named after the International Standard Organization.
The light sensitivity may be increased or decreased by manual adjustment, or by operating your camera using the auto ISO setting.
One of the ways your camera deals with varying light (in auto ISO) is by adjusting the ISO level.. The lower the ISO number, the less sensitive the sensor, or film,is to the light. The higher the ISO number the higher the sensitivity.
In other words, adjusting the ISO to a higher value (200 to 800 for example) will help you capture images in lower light situations… and without having to use a flash.
A caution about increasing ISO however: it will begin to add varying amounts of grain or “digital noise” to your pictures, depending on the just how high you set the ISO.