This bugged-out exclaimation to express utter joy and happiness. It's the upper-class twit version of booyah. Imagine Thurston Howell III from Gilligan's Island saying "booyar" with clenched teeth, a martini glass in one hand and Mrs. Howell's lingerie in the other.
As far as video games go, usually you say booyar when you find a rare cool-@$$ item or get alot of money. Money found in a booyarlean moment is sometimes referred to as MOOHLAR (like moohlah).