1. Queen Elizabeth and Prince Philip, and the other members of the English royal family, are members of high society. They are upper crust.
2. Only people with lots of money and the right social connections go to that university. It’s a school for the upper crust.

The expression can be used either as an adjective (opt.1) or as a noun (opt.2 ). Dating to the mid-19th century, this expression comes from the belief that the upper crust of bread was the best or most desirable part.