Poker-specific term. In one sense it's a loan, but really it's more like "investing" in a player, giving them the money they don't have to buy into a game ("table stakes") in exchange for a percentage of their winnings.
I've often heard it used to mean buying a player's buy-in, in return for (hopefully) a percentage of their winnings. Often there's no obligation to pay the staker back in the event of a loss, but in the case of a large stake to a mobster, maybe it's a different story
