As you have seen, each of the two halves of camera exposure control an aspect of film exposure. These two aspects do not work alone, but affect each other very dramatically. On the shutter speed selection dial there are the settings that can be made faster or slower, and on the aperture there are settings to make a larger or smaller hole for the F-stop. Moving either one of the two types of settings will either double or halve the amount of light striking the film.
In the case of shutter speed, you have to remember that the settings are fractions of a second. If you move from the setting 125 to the setting 250, you halve the amount of time the camera is allowing the light to strike the film. It's a simple case of doing the math ( 1/2 x 1/125 = 2/125 or 1/250 ).
However, moving the other way, from 125 to 60 is doubling the amount of light ( 2 x 1/125 = 1/62--reset for the camera as 1/60 ).
In F-stop, moving the setting from one to another will either double the size of the hole or halve the size of the hole. Thus, moving from one setting to another will either double or halve the amount of light being let in.
If your F-stop is set to F5.6, which is a large hole, and you change it to F8, you are halving the size of the of the hole since every movement from F2.8 toward F22 is making the hole get smaller and smaller.
Similarly, if you move from F16 to F11, you are doubling the size of the hole since every movement from F22 towards F2.8 is increasing the size of the hole.
So, as you can see, moving just one setting, be it F-stop or shutter speed, will double or halve the amount of light.You always want to strive for a balance of exposure. So let's say that the light meter calls for an exposure of F5.6 at 125 to give you the correct exposure. But you don't want a lot of depth of field--so you move the F-stop down to F4, thus doubling the amount of light being let in.
To compensate for the increase of light being let in, you cut down the time the light strikes the film to create the balance once more.
Since the F-stop movement doubled the exposure, to achieve balance you should halve the exposure by changing the shutter speed to 250. So, as a general rule of balance, moving one setting necessitates that you move the other setting in the opposite direction the equal amount of change.