When, the shutter opened and closed very quickly, the camera could actually stop the motion of objects that were moving in front of it. If the shutter opened and closed very slowly, the motion of moving objects would not be stopped and they would become a blur. This was not especially useful if the intent was to examine moving objects in detail.
This is what a camera's leaf type shutter looks like:
There are various leaves inside the shutter that open and close when the shutter is tripped. There are also "focal plane" shutters, which are actual, small curtains that open and close from side to side within a camera. Leaf shutters can be mounted on the lens itself whereas focal plane shutters are usually mounted on the camera.
So, the speed with which the shutter opens and closes determines how much the action will be stopped, and how much light is let into the camera. The aperture controls how much of the lens will be used to make the exposure. A small aperture uses only a small part of the lens and a large aperture, the opposite.
Both shutter speed and aperture work together to make a good exposure in which the subject is clear and the movement has been captured (stopped).
Here's an image I made recently using a slow shutter speed:
The subject is standing very still, so he is sharp, but the people around him are moving so they registered as blurs. This image was made with an exposure of f/5.0 and a shutter speed of 1/10th of a second. The camera was placed on a tripod with a weight attached to it to make sure there was no camera movement. Only the passersby were moving.
This image was made with an exposure of f/5.6 at 1/125th of a second. All action has been stopped.
This image was made at f/5.6 at 1/125th of a second. Note how all the action is stopped and everything is sharp. The faster the shutter opens and closes, the more action will be frozen in the image. The slower the shutter speed, the more blur is introduced in the image by moving objects and people.
So, the shutter is a powerful tool that can be used to alter the images we see in the camera for various, creative purposes.