Moulds are artifacts that can be used on armors , weapons , shield , bow and even quiver. They have various effects on your combat characteristics. Some parts of armors have characteristic symbols which you can't get on other parts of gear. Also for same part of armor , you can get different symbol at lvl 6 cuirass and lvl 11 cuirass , because symbols you get also level dependant.
Moulds can be bought at premium shop or at square of fire -> city fair (when the magic mould event comes)
The way they work is , you simply click to use and select which piece of gear you want to use on. If you have symbols on it already , next time you use mould it will replace previous symbols. Also it is pretty much random the symbols you get each time.
Additional tip is better the quality (color such green,blue,red,purple) of gear , more symbols it can get at a time. Like Green (2) , Blue (3) , Purple and Red can hold up to 4 symbols at a time.