ISO Speed was, in the “olden days”, the sensitivity to light of the roll of film you loaded in the camera. Each roll of film had an ISO Speed. The higher the speed, the more sensitive to light the roll of film was. A low speed of 100 was great for bright daylight. 200 was a good general purpose speed for most uses. As you went up in the ISO Speed range, 400 was the typical black and white speed for general use. A Speed of 800 was used for low light conditions.