What is ISO?
ISO measures the sensitivity of the image sensor. The lower the number the less sensitive your camera is to light and the finer the grain. Higher ISO settings are generally used in darker situations to get faster shutter speeds.
Although most people leave their cameras in Auto ISO, 100 ISO is generally accepted as normal and will give you lovely crisp shots- no noise.
ISO is an important element of digital photography, and so you must experiment with the three elements of the exposure triangle to understand their impacts.