Shutter speed refers to the length of time that a camera's shutter is open when taking a photograph. It determines how much light enters the camera and ultimately affects the exposure of an image. A fast shutter speed means that the shutter opens and closes quickly, allowing less light into the camera and resulting in a brighter picture with less motion blur. Conversely, a slow shutter speed means that the shutter stays open longer, letting more light in and potentially causing motion blur or other visual distortions if there is movement within the frame during exposure.