I'm not sure, either.
"You are shoe"
"You are a shoe" / "You're a shoe"
I went to google and typed in this phrase - it doesn't appear that it is a phrase that anyone uses...it is meaningless and strange.
Hope this helps in some way..?
I doubt that is a saying, It could be an original construction, although with the huge popularity of friends anything that was an original construction in that show might easily become a saying. I think when she says ¨it´s a metaphor¨ she is explaining that it was a metaphor that she made up, her own idea, not a saying from someone else.
To me the idea works even if you substitute different articles than things you wear. For example: All of my life, everyone has always told me, 'You're a cat! You're a cat, you're a cat, you're a cat!'. And today I just stopped and I said, 'What if I don't wanna be a cat? What if I wanna be a- a bird, y'know? Or a- or a horse! No, I'm not saying I want you to buy me a horse, I'm saying I am a ho- It's a metaphor, Daddy!
"You are a shoe" is a slang from the American drama "friends" according to this entry I found---