What is the meaning of the shutter speed of a camera?

The shutter is similar to a door. It admits light into the camera. Shutter speed is how long light is admitted to the lens. Because it is a fraction of seconds, the higher the number is, the faster the shutter speed and the less light that gets in.

A shutter speed of 125 is 1/125th of a second, much faster than, say, 60, which is 1/60th of a second (which is a good starting point for hand held shots without a zoom or image stabilization to prevent a blurry image).