The Noble Game—or simply the Game, as it's known—is Waterdeep's incarnation of the ages-old game of social power, political maneuvering, and societal machinations.
All nobles play the Game, whether they intend to or not. Indeed, the actions of those most contemptuous or abjuration of the Game can often influence it all the more. In simplest terms, the Game is about which House has the most social standing, relative to the other Houses. It is social currency, and those at the Crown of the Game are wealthy in it no matter their worldly coin.
The board upon which this Game is played is the Waterdhavian Social Season, a series of parties and other social events intended to permit the Houses opportunities to show off, show up their rivals, and in general make an impression.