ISO measures the camera’s sensitivity to light. Doubling the ISO number, doubles the camera’s sensitivity to light. However, increasing the ISO also decreases the image quality (or increases the ‘noise’).
When would you increase ISO then? Increasing the ISO is invaluable when shooting hand-held in low light as it allows you to use a faster shutter speed & therefore, avoid camera shake. However, if you’re using a tripod this isn’t necessary & keeping the ISO setting at ISO 100 is usually the best option (the longer shutter speed will make up for the lack of light). Similarly, if you are using a flash, high ISO settings are not required (although increasing the ISO will increase the effective range of your flash).