Casts are used with the Smeltery to make metal tool parts. To start, take a stone tool part, (ex., pickaxe head) and put it in a Casting Table hooked up to your Smeltery. Then pour some Aluminum Brass or Gold into the table. It will form a Cast around the tool part. You then remove the stone tool part, and are now free to pour any metal you like into the newly created Cast.
There are also clay versions of cast which act the same way as the normal cast however they are one use items.