ISO is, in an extremely simplified way, helping raise or lower brightness in your image. Lower ISO values, lets say 100-400, are general used in situations where lighting is very good. Situations like direct sunlight, or other well-lit scenarios. Higher ISO values, 800-6400 (or higher depending on your camera), are normally used when lighting is low to poor. For example, dusk, indoor lightning, concerts. Lower ISO values have a cleaner image if used correctly with aperature and exposure. Higher ISO values will create "noise" in the image; that grainy, red speckled image that occurs when the sensor is working hard to process the image. Hope that helps explain it somewhat!