The film speed tells you how sensitive it is to light. Lower numbers (ISO 50, 100) are less sensitive, so they need more light and are good for outdoor, daylight shots. They are considered "slow" speed film. High speed film, like 400, 800 and higher, are more sensitive to light so it takes less light to make a good exposure. These are often used for indoor shots or night shots. 200 is a medium speed and fairly flexible.
This isn't to say that you HAVE to use 100 for shooting outdoors, or 400 for indoors. Depends on what you want to do and what your camera can do. For example, I have some cameras whose shutters only go to 1/250. I would rather use low speed film for that so I have more flexibility in setting the cameras shutter and aperture for the exposure. If I use higher speed film, I have less flexibility - it's more sensitive to light, so I'd generally be using faster shutter speeds, but if I only can go as fast as 1/250, I have fewer options for shooting.
Make sense?