The shutter is basically a little door that opens and closes really fast to let in a certain amount of light. The faster it goes, the less light and vice versa.
This is the thing that annoys me the most. When you try and catch an action shot it either comes out blurry with a low speed or too dark with a high shutter speed.
To fix this problem you should use the other settings, ISO, the Aperture and change the environment, turn on lights/open window shades or go outside. As we talked about before a lower aperture number (bigger aperture) and a higher ISO will get you brighter shots so you can lower your shutter speed to catch some movement.