Have you ever seen a pig with a gold ring in his nose? The ring is put there to keep the pig from "rooting" or digging.
When you think of a golden jewel, you usually think of something and beautiful. Something that shines and glitters in the sunlight. A jewel of gold in a pigs nose?? Not so beautiful any more. It may have been beautiful at one point, and it could still be cleaned up and made to sparkle again, but it will never be even half way "beautiful" as long as it's in a pig's nose!
"Discretion" means having the power or right to act according to one's own judgment.
The word "fair" in this verse means "beautiful". Solomon is comparing a beautiful woman who is without discretion to that golden ring. It may have been beautiful at one point, but not any longer - not in a pig's nose!
A beautiful woman is only as beautiful as her actions allow her to be. Just like that golden ring is not pretty in a pig's nose, a beautiful woman is not beautiful when she's not demonstrating discretion, or using her judgment to act appropriately.
Can you think of some ways that you can or should show discretion?