It's pronounced err. It means a proto version of something, or first, as mentioned. So Ur-Gold is the original gold or the first gold. Then we have Grimnir, the guy they follow. He was an Ancestor God, one of the first Dwarves; an Ur-Dwarf if you like. He was shattered into pieces following a battle in the new Mortal Realms. Now we have these slayers who will do basically any depraved act to try and acquire Ur-Gold. Ur-Gold is Grimnir.