If you are shooting film, most often you will shoot at either 24fps or 30fps. Unlike video, the reason for this is that each of the 30 times that a frame is exposed, the camera shutter opens to expose the film and then closes while the film is advanced the next frame. Half the time the shutter is opened, and half the time it is closed. Once you have set the proper shutter speed on the meter, you must then set the meter for light sensitivity. The light sensitivity rating, or ISO rating, is a numerical value that rates how sensitive the film stock is to light.
Some films are extremely sensitive to light and require little light to make an exposure.
If ISO numbers confuse you, think about the film we all use to shoot snap-shots at home. Must people today are using an ISO 100 or 200 film. The major difference between these films is that the ISO 200 film is twice as sensitive to light as the ISO 100 film. ISO 50 film is only half as sensitive as ISO 100 film. Generally the size of the grain in the film affects how quickly light is absorbed.