Rah = Someone who went to private school (or wish they had), likes rugby (not football), probably a member of a rowing, sailing or yachting club, refer to their parents as "Mummy and Daddy". Boys: often seen to be wearing a lemon (or any pastel shade) sweatshirt slung casually over their shoulders and deck shoes.
Girls: Dress similar to that of boys along with pearls, numerous shopping bags and Daddy's credit card.
Names often include: Rupert, Will, Guy, Jeremy, Emily, Alexandra, Henrietta, Harriet. Followed by a double/triple barrelled surname, e.g. Harrington-Smythe.
Say Urban Dictionary.
Basically rich, privately educated, Jack Wills and Uggs wearing, posh, think they're above everyone else. Generally one to avoid.