It is made up of a glass tube sealed at the top. It contains mercury and the opened base of the tube dips into a beaker and below the surface of the mercury in the beaker.
Atmospheric pressure pushes down the mercury in the beaker which pushes the mercury up the inside the tube.
In the space above the mercury in the tube is a vacum, so nothing is pushing down the top of the mercury in the tube and atmospheric pressure will push the mercury up until the pressure of the column of mercury balances the atmospheric pressure.
The height “h” is approximately 760mm or 76cm of mercury.