Imagine a dark room with a window covered with blinds, the blinds are like a shutter, it opens up to let light in.
Shutter speed is how long the blinds stay open for. The longer the blinds stay open, the more light gets into the room. One of the reasons a picture can turn out blurry is if a person moves while the blinds are still open.
One of the things that determines how much light reaches the sensor in your camera is called aperture. The aperture is like the window. The bigger the window, the larger the aperture, which means more light gets in. More light means you can see the room better. The size of the aperture is measured in f-stops. The smaller the f-stop, the larger the aperture.
The ISO value is how a flim reacts to light shining on it. Some film is very sensitive to light and some are not.
Smaller aperture makes more of the picture stay in focus. If you want to take pictures of things moving fast(sports photographers) then you want quick shutter speed. Quick shutter speeds require smaller f-stops(larger aperture) to let more light in. If the aperture is not large enough and the shutter speed is too fast, the picture would be dark because it did not let enough light in to capture the picture.