Shutter Speed is how quickly the shutter of the camera opens and allows light to hit the sensor. I’m sure you’ve seen images where a person is jumping in the air, but they’re completely frozen in time, no motion blur; as well as images of the stars at night where they’ve turned into a swirl with trails of light. Those are two totally opposite settings.
To use the example of a jumping person, you would use a high, or fast, shutter speed, such as 1/500 of a second.
For the next example of a star trail, that is a very slow shutter speed, even up to several minutes! Typically, cameras only have settings for shutter speeds as slow at 30 seconds (often shown as 30″ on the display).
Shutter speed affects the brightness of the image as well as motion. The longer the shutter is open, the brighter the image will be. Think of it like a magnifying glass with the sun. The longer you hold the focused sunbeam over a single point, the hotter it becomes because so much light is hitting it. If you just pass the point over your skin, you might not even feel anything. If you hold the light to your hand, you’ll get a burn. The longer the shutter is open, the more light hits the sensor, and the brighter your image becomes. This is why photographers will sometimes refer to brightness in images as “hot.”
The rule of thumb is that 1/60 of a second is the slowest shutter speed to use for hand-holding a camera.
Summary: Slow shutter speeds = brighter images with possible motion blur. Faster shutter speeds = darker images with less motion.