A hallmark is used to guarantee the buyer that his piece of jewellery obtains certain minimum requirements. That means, that the standards for gold, silver and platinum are guaranteed. 
A hallmark consists of three punch marks: 
One is a set of letters which identify the company or person who made the piece. 
Two tells you the standard and material the piece is made of. 
Three is an official punch mark that guarantees, that this piece of jewellery has been inspected by office. 
 

In hallmark offices all jewellery-pieces with more than 2 g gold or platinum or more than 10 g silver are market with official puches. An official puchmark guarantees the consumer, that this certain piece of jewellery has been inspected and was found to obtain certain minimum standards. The minimum standard for gold is 585, for silver 800 and platinum 950. 
If any piece of jewellery falls short of these minimum requirements, it is prohibited to sell it in Austria. Antiques are not affected by this laws. 
These laws were first introduced under empress Maria Theresia in 1774, because  some goldsmiths tried to sell jewellery-pieces made of inferior alloys.