The ISO speed sensitivity is the level of sensitivity for your camera to available light of your photos. The higher the ISO number, the higher the sensitivity of your camera.
Low ISO: It is the ideal case and you should always try your best to use a low ISO if you can. For example, when there is enough light in the environment, then use a low ISO.
High ISOU: In some cases though, you will need a higher ISO. This will be the case if there is not enough light for your camera to capture the subject of your photo.
Actually, you may employ the Auto ISO of your camera if there is such function.