This is like the distance you have to travel, because it is the one thing you cannot change. On a camera you adjust time and speed because they work in proportion to each other. On some cameras you can adjust just one of the two and the camera will automatically adjust the other. This is usually referred to as aperture or shutter priority. Each faster speed is twice as fast as the last and each slower speed is half as fast as the one before.
Speed is the opening or aperture of the lens.
Just remember big is small and small is big when it comes to apertures and f- stops. The bigger the lens opening the more light that can hit the film in a shorter time. First of all, you want your film to be properly exposed. No one likes dark, grainy or washed out looking photos. Second, you want whatever you take a photo of to be in sharp focus.