'You wouldn't understand' refers to the meme: It's a _____ thing,
you wouldn't understand.
It's a (insert anything you can think of: 'Person/First Name' 'Place/Jersey' 'Thing/Jeep') thing, you wouldn't understand. If you can think of it, it's been inserted into this meme.
The original phrase was "It's a
black thing, you wouldn't understand". I could not find when it originated, but the earliest found it mentioned was in a book published in 1991 by Eugene Victor
Wolfenstein, a social theorist and psychoanalyst, titled 'The Victims of Democracy: Malcolm X and the Black Revolution'.