ISO refers to the camera’s sensitivity to light, and it has its origins in film photography, where different speeds of film had different sensitivities.
ISO settings on digital cameras typically range from 100 to 6400. Higher ISO settings allow more light into the camera, and they allow the user to shoot in low light situations. But the trade-off is that, at higher ISOs, the image will start to show noticeable noise and grain.
ISO should always be the last thing that you change, because noise is not desirable! Leave your ISO on its lowest setting as a default, only changing it when absolutely necessary.