Basically a camera ISO indicates how sensitive the sensor is to capturing light. The higher the ISO number the more sensitive to light.
On the upside ISO can be set by the user to increase/decrease the amount of exposure. Doing this can affect the brightness or darkness of the image. This is especially beneficial in trying to capture images in low light situations.
On the downside a higher ISO setting can affect the image quality by causing an increase in the overall ‘graininess’ or ‘noise’ and lower the contrast of an image. In general, the higher the ISO is set the more this will affect the image. Modern camera sensors and processors are continuously showing improvements in high ISO image quality.
Due to this trade-off it is recommended to adjust the overall brightness by adjusting shutter speed and aperture combinations to avoid a higher ISO if possible.