Shutter speed is the speed that the shutter opens, lets in the image and then closes. Shutter speed manages two elements of your photo: the amount of light in your image and the amount of motion captured in your image.
When the shutter speed is on a lower/slower setting the shutter is open a longer amount of time capturing all the movement found in your scene. The slower the shutter speed the longer the shutter is open and the more movement that is captured in your image. When the shutter speed is on a higher/faster setting the shutter is open less time capturing a smaller amount of "time" in our scene, stopping any movement that is happening. The faster the shutter speed the less time the shutter is open and the less motion is captures. The higher/faster the shutter the sharper your image. When capturing your Christmas tree you want to capture the "movement" of the lights so you would select a lower/slower shutter speed.