Among three Pillars of Photography APERTURE, SHUTTER SPEED AND ISO , Iso also plays a significant role in Photography. Iso is simply the Sensation of the camera to the light. Iso range basically starts form 100 and is upto 12800. In some latest camera it starts from 50 and can range upto 102400.
Lower the Iso the camera is less sensible to the light and image will be darker.
Higher the Iso the camera is more sensible to the light and image will be more brighter.
Lower Iso gives you the higher quality Image with less noise in the image(with no grains) whereas Higher Iso gives you the Lower quality image with more noise(with grains). So, lower Iso is prefered for the best quality image if you have sufficient light in your environment. You can use lower Iso in medium light as well if you lower down the Shutter Speed and Aperture Number from your Camera Setting, but for this you must have proper Knowledge to Balance ISO, APERTURE and SHUTTER which is Known as EXPOSURE TRIANGLE.