What is meaning of unit (mg-Pt/cm^2) in relation to fuel cells? I do not understand what the meaning is of the unit 

mg-$\ce{Pt}$/cm$^2$ 

which I have come across in reading about the theory of fuel cells.

In simple terms, what does this unit mean?
The unit in question means milligrams of platinum per square centimeter. 

In your application - fuel cells, and specifically the membranes used to fabricate them - it describes how much of the metal is present per square centimeter of membrane.