Shutter speed refers to the duration that a digital camera opens the shutter when shooting a photo. The shutter is part of a digital model which opens to help the image sensor receive the light when you hold down the shutter button.
Commonly, the shutter speed is represented by fractions of a second, for example, 1/1500th of a second. When you shoot photos under inadequate lighting conditions, the shutter speed needs to be slower as the longer the shutter opens, a larger amount of light can come into the camera sensor to produce photos. For shooting subjects that are moving fast, the shutter speed need to be faster, so you can avoid the image blurriness.