It's a two-player game, played on a board depicting twelve islands. Players use cards to place bridges between these islands or remove opponent's bridges. If you get the majority of bridges around an island, you place one of your marker stones on it and also remove any of your opponent's bridges to that island – which might cause them to lose a bridge majority on an adjacent island and lose a marker stone there.
Don't just take our word for it! Check out this review of Kahuna.