For Dexter, Judy represents all that glitters. That is, she is the embodiment of ambition and success, money, and beauty.
That's what Dexter wants, and that's why he never sees her as she truly is. Judy is just human, but Dexter sees her as perfect, as the key to his happiness. She isn't, and Dexter's bubble is burst at the end of the story when he finally realizes this.
Let's take a look at some evidence for this idea that Judy represents all that is desirable to Dexter: beauty, riches, and success.
First, let's note that Dexter is highly ambitious, even in his youth. He doesn't even have to work as a teenager, since his dad makes enough money to support their family, but Dexter works anyway--day in and day out, he caddies at the golf course, all for a few dollars a day. He wants the "pocket-money," or the money he can have on his own to spend.
More evidence of Dexter's ambition and success appears when the narrator informs us of it directly, and when we see Dexter climbing higher in the social and financial hierarchies:
- "He wanted not association with glittering things and glittering people--he wanted the glittering things themselves. Often he reached out for the best without knowing why he wanted it--and sometimes he ran up against the mysterious denials and prohibitions in which life indulges."
- "He made money. It was rather amazing."
- "Before he was twenty-seven he owned the largest string of laundries in his section of the country."
- (Spoken by Dexter to Judy) "I'm probably making more money than any man my age in the Northwest."
So we know that Dexter wants to possess wealth, specifically "the glittering things" in life, and we know he wants to possess Judy, although she continually torments him. (She's constantly drawing him in, then pushing him away.) Dexter realizes that he can never truly have her, but he still desires her because for him, she's not just the epitome of beauty, but also his loftiest, most unattainable goal in life.
Why would we say she represents money then, or "all that glitters"? Because, in the story, Judy literally glitters.
Every time her beauty is described in the story, you get a mental image of a figure bathed in golden light. Here's the most relevant of these images:
"Judy Jones, a slender enamelled doll in cloth of gold: gold in a band at her head, gold in two slipper points at her dress's hem. The fragile glow of her face seemed to blossom as she smiled at him."
She's dressed in gold, wearing a halo, and glowing. She glimmers. That image of Judy as something that glitters, and as a creature who's more than human (part angel and part doll) helps support the idea that she represents success, beauty, and money for Dexter, which is what he wants.