The shutter on a camera is a sophisticated piece of electronics that precisely controls the amount of time the sensor is exposed to light. The duration for which the shutter remains open is called 'shutter speed'. In photography speak, shutter speeds are said to be fast (shorter duration) and slow (longer duration). A shutter speed of 1/200 seconds is faster than a shutter speed of 1/60 seconds (remember fractions :)).
"Clicking" activates the electronics that controls the shutter. As soon as the shutter is open, ambient light starts accumulating on the photographic medium (film or digital sensor) and continues to do so until the shutter closes again. This means that by controlling the shutter speed we can make a bright, sunny day look grey and cloudy (using a very fast shutter speed) or make a pitch-black night feel like dusk
