These two elements are combined to form a mirror.
The glass plate is flat and smooth, and a thin film of silver, which reflects light well, is applied to it. Light is reflected a little on the surface of the glass, but on the silver-plated back side, light is almost 100% perfectly reflected. This creates a mirror that reflects light very well.
Thus, a mirror is simply a glass plate with a thin layer of silver on it, but it has a very long history.