Your camera’s ISO setting determines how sensitive the sensor is to light. It acts a bit like the gain control on a guitar amplifier, boosting the strength of a weak signal. As a result, it can enable you to take pictures when light levels are low (a weak signal) and you can’t (or don’t want to) use a larger aperture or a slower shutter speed to make a usable exposure. The downside to increasing the ISO is the same as turning the gain control on an amp: more ‘noise’. Nikon D-SLRs typically have an ISO range that runs from ISO100 to 6400, with a number of ‘Hi’ settings. For the most part you’ll want to stick with lower sensitivities of ISO100 to ISO800 for the ‘cleanest’ pictures, as the higher sensitivities will look grainy and be covered by coloured speckles. You can increase the noise reduction (Shooting Menu>High ISO NR) to combat this, although this will also smudge fine details.