Congratulations on upgrading from digital to proper photography!
Yes, a film speed of 100 ASA is equivalent to a digital camera having an ISO setting of 100: the ISO setting of the digital camera explicitly makes it behave as if it worked with film of that speed. What you need to understand is what that actually means.
The film speed tells you how much light is needed to expose the film properly. Faster film has higher sensitivity, so it needs less light. Two factors determine the amount of light that is supplied: the length of time the shutter is open, and the size of the opening. For normal photography, the shutter time is given as a fraction of a second, so when the shutter dial says 125, that's actually 1/125th of a second. The size of the opening is given as a ratio between the focal length of the lens and the diameter of the opening (or aperture) as seen from the front of the lens. When the aperture ring is set at 16, that means f/16, where f is the focal length of the lens. Thus, if you're using a 90mm lens set at f/16, the visible opening as seen from the front has a diameter of 5.6mm, or about 7/32".
Using the ratio of the focal length as the aperture specification has the cool advantage that, say, 1/125 of a second at f/16 gives the same amount of light to the film regardless of what focal length lens you've got. Switch to a 50mm, or a 180mm, or whatever, and the correct setting will still be the same. On the down side, we have to accept that while the shutter speed is proportional to exposure (1/250th of a second gives half as much light as 1/125th does), the aperture numbers are related to the square root of the exposure.
The aperture scale typically has the numbers 22, 16, 11, 8, 5.6, and 4, and maybe a couple more at either end. Because these numbers say something about the diameter of the opening, while it's the opening's area that determines how much light gets through, a change from f/16 to f/11 means twice as much light reaching the film, and going from f/16 to f/8 means four times as much light. Luckily, lenses are marked in such a way that one full stop (going from one printed number to the next) either doubles or halves the amount of light let through. If you look closely at the scales on your RB67 lens, you'll see that when it's set to 1/125th and f/16, there are lines connecting the two rings, showing that this is equivalent to 1/60th and f/22 (doubling the time and halving the opening), and to 1/250th and f/11 (halving the time and doubling the opening).
So what about that film speed? Well, again, the number is proportional to the amount of light needed. 100 ASA film needs four times as much light as 400 ASA film does. (And that's two stops, or, for instance, going from our 1/125th at f/16 example with 400 ASA film to 1/60th at f/11 with 100 ASA.)
You say Kodak specifies 1/125th at f/16 for their 100 ASA film. That's what we call the "sunny 16" rule, which says that outdoors on a sunny day, you'll want to set your aperture at f/16, and the shutter speed at the reciprocal of the film speed, or the nearest available setting. Thus, with our RB67s, 1/125th for 100 ASA film, and 1/400th for 400 ASA. If I recall correctly, the instructions packaged with the film also give suggested aperture settings for other weather conditions. If not, you'll easily find them on the web. Ideally, of course, you'd want to use a light meter. You can buy a hand-held one, or you could use your digital camera, set at the ISO setting of the film you're using in the RB67, to find proper settings.
-tih
