The letters ISO on your digital SLR camera refers to film speed. Even though your camera is most likely not film at all, but rather digital, the ISO camera setting still has the same function as older film cameras. ISO determines how sensitive the image sensor is to light.
Depending on your digital camera model you may also have them in the range of 64, 100, 160, 200, 400, 640, 800, 1600 and higher.
The lower the ISO number, the slower the speed. The higher the ISO number, for example 1600 the faster the speed.