[ okay because you're a king and all... he's going to explain this slowly. only once though. ]
You know what gold is, obviously. It's melted down and combined with a metal much stronger than steel. It makes it nearly impenetrable. More than that, it keeps its shape better than any other alloy.
An alloy is just two substances mixed together. Basically. [ there's more to it but he's not trying to dumb things down so much as put them in terms arthur might grasp. let him make you armor let him make you armor ]