As I said, the shutter speed is comparable to the pen speed when you write. Faster means less ink, in photography less light.
The term refers to the physical composition of a normal SLR (digital or analogical), where there is indeed a component called shutter. However I’m not explaining here the internal mechanism of a camera, I’ll do later in an advanced chapter.
The light enters the lens and reaches the digital sensor that is impressed. Through the shutter speed, you can determine how long to let the light “write” on the sensor.
Depending on the conditions of light, there will be a time value which will give the sensor the right amount of light. If the sensor will not be impressed enough the photograph will be underexposure and the result will be a dark picture, whereas if the sensor will be reached by light for a period greater than necessary, the phoograph will be overexposed, and the result will be a bright picture.
It’s impossible to say what is the “right” the shutter speed. Almost everything depends on the ambient light in which we are photographing; outdoors in a sunny day we can get properly exposed pictures even staying well below 1/200 of a second, inside a building this will be much more difficult and we may use longer shutter speed up even a few tenths of a second if we do not directly light the subject of the photograph using a flash or other lights.