This is the amount of time the shutter is open, allowing the light to get to the sensor or film. The longer a shutter is open, the more light can fall on the sensor. Speeds can be as fast as 1/8000th of a second or as slow as 30sec. Any shutter speed lower than 1/60th could suffer from camera shake, where the picture will be blurred, unless you use a tripod or rest the camera on something solid. 1/125th is a good speed to start with.