I just started with film photography and I have an expired 200 ISO film roll. Some people on the internet said that I should change the film speed on the camera to 100 to "overexpose" it. But I want to understand more how does this work. What does the ISO dial actually do? Does the dial tells the sensor how much sensitive to light should it be? If that so, why ISO 100 is "overexpose" but not "underexpose"?
First there is no sensor in a film camera. (Some, but not all, have light meters)
Films are made with different sensitivities to light and are classified as Film Speeds. A film with a 200 ASA film speed is more sensitive to light by half as much then a 100 ASA film. (meaning it requires less light, half as much)
You set up a shot in a room that has a light source that can be adjusted. You place your camera on a tripod and your subject under the light source. You set the light source to low power. You put 200 ASA film in you camera and you set the ASA dial to 200. You take a meter reading of your subject and it tells you you need 250ths of a second shutter speed and F8 for an aperture to give you a correct exposure. You set your camera settings and take the photo.
NOW you change the lights source so it is is putting out twice as much light than it did in the first shot. You now load 100 ASA film and set the ASA dial to 100. You do a measurement and it now tells you you need 125ths of a second shutter speed and F8 to give you a correct exposure. You set your camera settings and take the photo.
The two photos should look the same, have the same exposure value. 125ths of a second increases the amount of light entering the camera by double of what 250th of second lets in and in the two photos this looks the same because the 100 ASA film needs twice as much light as the 200 ASA film.
A film camera does not know what ASA film you have loaded into it, you have to tell it by setting the ASA dial on the camera to the correct ASA setting. If you put 100 ASA film in the camera than you need to set the dial to 100 ASA, this tells the light meter in the camera how to measure the light in the seen you are photographing and give you the correct shutter speed and aperture settings to get the proper exposure.
In a digital camera the sensor is the equivalent of the film. The sensor's sensitivity to light can be changed to different levels (ISO). The digital camera used what was the ASA dial on a film camera and repurposed it for ISO sensitivity adjustments.
Since the advent of digital photography the Term "Exposure Triangle" as been invented. The triangle being ISO, shutter speed and aperture. This term in my opinion is misleading to beginning photographer because it, implies, causes many to think that ISO is tool to effect exposure, ( the amount of light entering the camera. ) The confusion is, that many people think because it is lumped in with shutter speed and aperture in the "Exposure Triangle" that it changes the exposure. It effects the settings needed for the exposure, not the exposure itself.
It only effects exposure in so much as it changes the sensors sensitivity to light, it does not change the amount of light that is being exposed onto the sensor, only the shutter speed or the aperture size change the amount of light entering the camera.
If "Exposure Triangle" (specifically ISO) is not explained properly, and it rarely is, then it leads to confusion about the function of ISO. Many disagree with me on this point but the confusion is evident in many questions on this web site and others.
The ISO dial indeed gives information to the light meter. If set to ISO 200, the light meter will return an shutter speed / aperture / or both (depending on the mode) that will give a proper exposure at that sensitivity to light.
Colour and black & white negative film can handle a decent amount of overexposure. That is because the negatives will become denser (read: more silver halides will remain on the film base) when an exposure gets brighter. Thus, no information is lost, but besides an increase in grain, there will be other drawbacks. With underexposure, you would end up with thinner negatives and thus a loss of information. It is for this reason reversal film (slide film) will actually do better with underexposure, as here you get a positive image on the film once it is developed.
Anyway, if you set the dial to ISO 100 while actually shooting an ISO 200 film, the meter will assume a less sensitive film has been loaded and therefore exposure times will be twice as long than would be 'appropriate' for the ISO 200 film. This is frequently done with colour negative film (especially Portra), to get a fairly flat image with not very deep shadows. Whether that is something you want to try is up to what kind of images you want to obtain.
Additionally, since you mention your roll being expired, exposing the film at a lower ISO than at box speed is often done to counter a process called 'fogging'. This happens as film expires, where over time a dark cast falls over the entire film. The severity of this, and if it happens at all, depends strongly on how the film was stored. A rule of thumb is to expose +1 stop for every decade the film was expired for. So, if you have a 15 year old roll of film, expose ~ +1 2/3 than box speed. Do not do this with reversal film, however.