ISO is the third point in Exposure triangle. Higher ISO number means hisensor is highly sensitive to the light and vice versa. Higher ISO comes with one side effect - noise (grainy photos). Photo will not be crisp and clear with highest ISO number. Example - In day light, ISO of 100 or 200 would be suitable. In dark conditions, you will need to use higher ISO. To best understand the effect of ISO, take same picture with different ISO settings.