This proverb uses a vivid and somewhat jarring image to illustrate a point about the importance of inner character matching outward appearance. Let's break it down:

"Like a gold ring in a pig’s snout...": A gold ring is a beautiful and valuable ornament, meant to enhance the wearer's appearance. A pig, in contrast, is considered unclean, uncouth, and primarily focused on basic needs like rooting around in the dirt. Putting a gold ring in a pig's snout is absurd. The ring's beauty is completely wasted and out of place on such an animal. It's also somewhat repulsive to think about.

"...is a beautiful woman who lacks discretion.": This compares a beautiful woman lacking discretion to the gold ring in the pig's snout. "Discretion" can mean several things in this context:

Good judgment: She lacks the ability to make wise and sensible decisions.
Prudence: She's not careful or cautious in her behavior and speech.
Modesty: She's not modest or restrained, perhaps being loud, boisterous, or overly revealing.
Self-control: She may lack the ability to control her impulses.

The meaning of the Proverb:

The proverb is saying that outward beauty is diminished, even rendered worthless or ridiculous, when it's not matched by inner qualities like good judgment, prudence, and self-control. In other words, a beautiful woman who behaves foolishly or immorally devalues her beauty. Her lack of discretion detracts from, or even negates, her physical attractiveness. The beauty is wasted, just like the gold ring on the pig.

Essentially, the proverb emphasizes that true beauty is more than skin deep. Character and wisdom are essential qualities that enhance and give lasting value to physical attractiveness. It suggests that inner beauty or character is more important and lasts longer than outer beauty. It's a reminder that beauty can be fleeting and deceptive if not accompanied by good character.