A half silvered mirror is used when there is a requirement that the glass separation allows light only from one side to pass through, so it appears as plain glass from one side but as a mirror from the other side.
To make this glass it is very lightly coated with atoms of silver. The coating is such that only half the light can pass through the glass and the other half is reflected back. When the glass is being used, the side from which the glass is intended to appear as a mirror is brightly lit while the room on the other side is very dimly lit.
This allows light from one side to pass through as it is brightly lit but from the other side no light passes through.