"Metall coating" means that the inner side of the outer pane of a double glazing unit is coated with metall.
Usually silver is used for the purpose. The coating is so thin that light can penetrate but heat is kept back. As a silver coated mirror would reflect light the much thinner coating reflects a certain wave lenght of energy-heat. It means that the inner space of a double glazing window pane is kept warmer, the surface of the inner pane would feel warmer when touching it. A not metall coated double glazing window pane would have an U-value (I guess the numbers from my memory) of about 3.5 whilst a metall coated double glazed window pane would have an U-value of 1.6, if you get additionally Argon or Krypton gas filling between the two panes the U-value will be reduced to 1.2 .
You could check the home pages of "pilkington" or " vistatherm" or another manufacturer, I think the SEI has also some information about the issue. The lower the U-value the less heat/energy gets lost, the warmer the surface, the less money it costs to keep the room comfortable . The metall coating also protects against UV light , this would help to decrease the colour fading effect of the sunlight. For example of fabric, curtains, pictures,flooring and our skin as well. You won't get brown, tanned behind such panes as quick as with non coated panes. But this is only a side effect, the aim is energy conservation.