This term is used to describe a hand that is not only the best hand at the table but is also the hand that was dealt the highest spade card face-down. That player would thus win the entire pot. When the game Black Mariah is called, it is a game that only ends when a single player has the best hand at the table AND is dealt the highest spade card face-down. If the best hand and highest spade are in two different hands, then everybody re-antes, and the game is redealt by the player to the left of the dealer, and replayed.
This continues until a single player has the best hand and highest spade face-down. One of the only ways that this game can be ended is by continually betting the maximum bet after being dealt the Ace of Spades (or a high enough Spade), bluffing the other players out of the hand. You may not have the best hand at the table, but players who do not continue to see bets are out of the game. When only one player is left in the game, the game is obviously over, assuming that one player has at least one spade card face-down.