ISO refers to how sensitive your camera sensor will be to the incoming light when the picture is taken. A high ISO setting allows the sensor to be more sensitive to light for shooting in low light situation, where as a low ISO setting allows the sensor be less sensitive to light for shooing in a better lighting situation. How this works is we set the level of ISO on the camera, and then the software inside the camera determines the level of light and incorporates this data to the overall photo. This is how a brighter or darker is produced.
Even though picture seems brighter in higher ISO settings, keep in mind that the lighting is only artificially enhanced. The actual lighting is still the same and a big disadvantage of having a high ISO is it increases the occurrence of “noise” in a photograph. What this means is a photo will look grainy if you set the ISO too high because the camera sensor is trying to compensate and guessing too much of what the dark area should be on a photo. So generally speaking, we should always be using the lowest ISO setting possible when taking photographs.