What Exactly is Meant by "X Puts Y to Shame?"

"X puts Y to shame" means "X is so good at something that Y, while normally good at that thing, looks terrible in comparison."

Gold is something with a lot of luster, but Lancelot is telling Guenevere that her face has much, much more luster than Gold -- not literally, of course. This is an example of hyperbole.

"X puts Y to shame" can also be used without hyperbole. If a famous runner is outrun by an untrained amateur, it could be said that the amateur put the runner to shame. Quite literally, he caused the runner to be ashamed by failing to be better.

In this context, the sentence means that "the luster of your face is so much more than gold".

'Put to shame' means 'embarrass'. In this particular sentence, the luster of your face is so much that, it would embarrass gold.

Some other uses are:

The veteran athlete put to shame many of his younger compatriots by his speed.

and

The fresh graduates were put to shame by the work done by the experienced developer.